feat(compact-basic): baseline compaction backend (squashed from compact-basic)

Collapses the per-round review churn of the prior compact-basic branch into a
single clean baseline on top of compact-interface, so the upcoming retention
refactor lands as fresh, well-scoped commits rather than stacking on a history
of fixes that are being superseded.

docs/architecture.md

@@ -192,7 +192,7 @@ Every MVP feature (including the TODO-marked ones), with the mechanism that impl
 | `/loop` | on `agent/turn-end`, `send()` the next iteration; or force-continue |
 | Dynamic workflow | orchestrator plugin on `agent/turn-end` / `agent/step-end` driving `send`/`steer` (+ sub-agents later) |
 | Queued + steering messages | core `Agent.send()` / `Agent.steer()` |
-| Context compaction (auto + manual) | the `ctx.compact` seam ([dsh-compact](../packages/compact/compact)): a backend summarizes an older surface range into a single `user/message` `replace` op, bracketed by log-only `compact/*` events; auto = check token pressure at turn boundaries, manual = a `/compact` tool. See the [compaction capability-seam RFC](rfc/proposed/feature/2026-06-18-compaction-capability-seam.md) |
+| Context compaction (auto + manual) | the `dsh-compact` seam (`ctx.compact`) + a backend (`dsh-compact-basic`) wrapping `agent/request`: a backend summarizes an older surface range into a single `user/message` `replace` op, bracketed by log-only `compact/*` events; auto = check token pressure before each model call, manual = a (deferred) `/compact` tool invoking the same `ctx.compact` routine. See the [compaction capability-seam RFC](rfc/proposed/feature/2026-06-18-compaction-capability-seam.md) |
 | System prompt configurability | `ctx.systemPrompt.section()` with ordering |
 | AGENTS.md (root) | a section provider reading the file |
 | AGENTS.md (subdir, on-touch) + file-change notices | `agent.inject()` from a watcher / tool-result listener |
@@ -220,6 +220,6 @@ Code skeletons for the three plugin shapes (tool, hook/permission-gate, UI) and
 Tracked here deliberately — each is designed-for but not implemented:
 
 - **Sub-agent spawn/fork semantics** (seam: `AgentLoop.create()`); inter-agent channels beyond `send`/`steer`/events.
-- **Compaction implementation** (auto thresholds, summarization prompts) on the `agent/request` seam, with its session-event types added by declaration merging.
+- **Compaction** — the `dsh-compact` seam (`ctx.compact`) and the `dsh-compact-basic` backend exist (auto thresholds, summarization on the `agent/request` seam, `compact/*` session events via declaration merging). The model-facing `/compact` consumer tool is still deferred. See [the compaction capability-seam RFC](rfc/proposed/feature/2026-06-18-compaction-capability-seam.md).
 - **Parallel tool execution** (concurrency-safety hints on ToolDefinition).
 - **Session branching/tree** (pi-style entry tree) if needed beyond seed-based forking.

docs/cordis-catalog/events-and-services.md

@@ -207,7 +207,7 @@ A session was created in the store.
 'session/created'(session: Session): void
 ```
 
-Source: [`packages/core/session/src/index.ts:33`](../../packages/core/session/src/index.ts)
+Source: [`packages/core/session/src/index.ts:34`](../../packages/core/session/src/index.ts)
 
 #### `session/event` — emit
 
@@ -219,7 +219,7 @@ An event was appended to a session log (sync, fire-and-forget). This is the per-
 
 Types: [SessionEvent](../core-data-structures/core.md)
 
-Source: [`packages/core/session/src/index.ts:39`](../../packages/core/session/src/index.ts)
+Source: [`packages/core/session/src/index.ts:40`](../../packages/core/session/src/index.ts)
 
 #### `session/flush` — parallel
 
@@ -229,7 +229,7 @@ Awaited durability checkpoint. The agent loop awaits `ctx.parallel('session/flus
 'session/flush'(session: Session): Promise<void> | void
 ```
 
-Source: [`packages/core/session/src/index.ts:48`](../../packages/core/session/src/index.ts)
+Source: [`packages/core/session/src/index.ts:49`](../../packages/core/session/src/index.ts)
 
 ### `subagent/*`
 
@@ -430,7 +430,7 @@ get(id: SessionId): Session | undefined
 list(): Session[]
 ```
 
-Source: [`packages/core/session/src/index.ts:321`](../../packages/core/session/src/index.ts)
+Source: [`packages/core/session/src/index.ts:322`](../../packages/core/session/src/index.ts)
 
 ### `ctx.subagents` — `SubagentService`
 

docs/core-data-structures/compaction.md

@@ -1,6 +1,6 @@
 # Compaction
 
-The compaction seam — a [capability seam](../rfc/implemented/architecture/2026-06-13-capability-seams.md) split like bash: interface ([dsh-compact](../../packages/compact/compact), `ctx.compact`), implementation (a backend such as `dsh-compact-basic`, deferred), and consumer (a `/compact` tool, deferred). Compaction is **one optional capability**, not part of the agent-loop spine — so its vocabulary lives here, not in [core.md](core.md). A tokenizer- or template-based backend is a sibling package implementing the same interface. Unlike bash, the interface necessarily depends on `dsh-session` and `dsh-llm`: its verbs are defined over a `Session` and its output is the `ContentBlock` vocabulary (see the [compaction capability-seam RFC](../rfc/proposed/feature/2026-06-18-compaction-capability-seam.md)).
+The compaction seam — a [capability seam](../rfc/implemented/architecture/2026-06-13-capability-seams.md) split like bash: interface ([dsh-compact](../../packages/compact/compact), `ctx.compact`), implementation (a backend such as [dsh-compact-basic](../../packages/compact/compact-basic)), and consumer (a `/compact` tool, deferred). Compaction is **one optional capability**, not part of the agent-loop spine — so its vocabulary lives here, not in [core.md](core.md). A tokenizer- or template-based backend is a sibling package implementing the same interface. Unlike bash, the interface necessarily depends on `dsh-session` and `dsh-llm`: its verbs are defined over a `Session` and its output is the `ContentBlock` vocabulary (see the [compaction capability-seam RFC](../rfc/proposed/feature/2026-06-18-compaction-capability-seam.md)).
 
 Source: [`packages/compact/compact/src/types.ts`](../../packages/compact/compact/src/types.ts)
 
@@ -11,7 +11,7 @@ Compaction extends [`SessionEventMap`](session.md) with three event types via de
 | Event | Payload | Role |
 |---|---|---|
 | `compact/start` | `{ turn }` | acquires the log-recorded lock |
-| `compact/summary` | `{ summary, shadowedRange, shadowedSeqs, shadowedTokenCount }` | provenance: the summary blocks, the shadowed seq range, and the estimated token count |
+| `compact/summary` | `{ summary, shadowedRange, shadowedSeqs, shadowedTokenCount }` | provenance: the summary blocks, the shadowed surface-boundary pair (`start`/`end` seqs — a position span, not a numeric interval), the shadowed seqs in surface order, and the estimated token count |
 | `compact/end` | `{ turn, error? }` | releases the lock (`error` set when summarization threw) |
 
 The lock brackets the **whole** operation: `compact/start` is appended first, then summarization, the `compact/summary` provenance record, and the `user/message` replacement all land, and only then `compact/end`. Releasing the lock last turns a crash mid-operation into a detectable orphaned lock (a `compact/start` with no matching `compact/end`) rather than a `compact/end` that falsely claims compaction finished.
@@ -32,9 +32,16 @@ interface CompactionResult {
   endSeq: number
   /** The summary content blocks produced by the backend. */
   summary: ContentBlock[]
-  /** The seq range that was shadowed [start, end] inclusive. */
+  /**
+   * The surface-boundary pair that was shadowed: the seqs of the first
+   * (`start`) and last (`end`) surface nodes of the replaced range. A
+   * surface-POSITION span, not a numeric seq interval — after a prior replace
+   * lands a fresh high-seq summary node at an older range's position, `start`
+   * can be GREATER than `end`. {@link CompactionResult.shadowedSeqs} is the
+   * authoritative set of shadowed nodes, in surface order.
+   */
   shadowedRange: { start: number; end: number }
-  /** The seq numbers of all shadowed surface nodes. */
+  /** The seqs of all shadowed surface nodes, in surface order. */
   shadowedSeqs: number[]
   /** Estimated token count of the shadowed content. */
   shadowedTokenCount: number

docs/module-graph.md

@@ -23,6 +23,10 @@ graph TD
   llm-replay --> llm
   llm-replay --> session
   session-persistence --> session
+  compact-basic --> agent
+  compact-basic --> compact
+  compact-basic --> llm
+  compact-basic --> session
   invariants --> agent
   invariants --> llm
   invariants --> session
@@ -106,6 +110,7 @@ graph TD
 | `compact` | `llm`, `session` |
 | `llm-replay` | `llm`, `session` |
 | `session-persistence` | `session` |
+| `compact-basic` | `agent`, `compact`, `llm`, `session` |
 | `invariants` | `agent`, `llm`, `session` |
 | `session-persistence-jsonl` | `session`, `session-persistence` |
 | `session-persistence-sqlite` | `session`, `session-persistence` |