fix(apiproxy): serve history events and projections from one log position

Review found two defects in the previous commit's ordering fix.

@pku-xht: `historyStateFor` copied the attached session's events, the handler
then awaited `presenterScopeFor`, and only then read the projection baseline
off the still-live Session. An append during that await served events cut at N
beside a baseline folded to N+1 — one response describing two moments. The
same restructure had also moved the baseline read outside the `try`, so a
failing snapshot escaped the structured `internal` error.

Both awaits now happen before the cut: `historySourceFor` resolves which
session serves the read, `presenterScopeFor` ensures the recorded composition,
and `historyCutOf` then reads events and baseline adjacently with nothing
between them. The whole sequence is back inside the try.

The invariant judged any scoped assembly with a chain of one as an unjoined
agent, which rejects a legitimate assembly in a standing preset key (that key
has no parent of its own). It now gates on `context.agent` — a scope-only read
is not an agent and is out of range by construction rather than by a premise
about who else calls `assemble` — and asks the roster's own
`composedPreset()` instead of introspecting chain length. The advisory warning
uses the same relation.

Also from review: the `2026-08-05-per-agent-tool-presentation` note still
described `presentAs` as per-agent, which standing mounts made false and this
branch's own rewording contradicts; the duplicated "process-wide unit table"
argument collapses to the Agent Note with pointers from the five copies; a
dead `.sort()` before `arrayContaining`; and change-history narration in the
tool-cordis README.

packages/bundle/web-app/cordis.patch.yml

@@ -301,16 +301,11 @@
   disabled: true
 
 # The token METER stays on the host plane; only the compaction backend that
-# reads it moves. It takes no configuration at all, keys every fold by Session,
-# and registers no tool or prompt section, so one host instance serves every
-# preset exactly as before. It also owns the `tokenUsage`/`contextPressure`/
-# `contextBreakdown` projection units, and the projection registry is
-# process-wide rather than scope-layered: registered from inside a preset, the
-# browser's context meter would appear for a session whose own composition
-# mounts no meter as soon as some OTHER session mounted one, and disappear
-# entirely in a process that only ever ran `minimal`. That is the same criterion
-# `tasks` and `goals` are read by — a Service the host and every preset can see
-# belongs to the plane both can see.
+# reads it moves. It owns the context-meter projection units, and that table is
+# process-wide, so preset ownership would make the meter a function of which
+# presets happen to be mounted rather than a per-session fact. Same criterion as
+# `tasks` and `goals`; the reasoning has one home in
+# `.agents/notes/implemented/architecture/2026-08-10-host-plane-ownership-after-presets.md`.
 
 - id: compact-basic
   disabled: true