Fix doc cross-links for the hierarchy; add package-path + shape gates

Merge brought in the RFC-classification reorg and two new doc gates;
rewrite every drifted packages/<name> cross-link (Markdown link targets,
moved-README relative depths, and .ts comment paths) to the grouped paths.

Add two doc-sync/hygiene gates so the manual checks this restructure
needed become automated:
- verify-package-paths.ts: flags a packages/<path> reference (in Markdown
  or a .ts comment/string) that does not resolve AND names a real package
  in a segment — i.e. a stale path to a MOVED package. A path naming a
  non-existent package (a forward-looking proposal) is left alone, so it
  applies uniformly across proposed/implemented/rejected.
- check-workspace-constraints: assert the packages/<group>/<pkg> depth-2
  shape (group dirs carry no package.json; no flat or over-nested
  packages). Group names stay open; only the shape is fixed.

docs/development.md

@@ -135,7 +135,7 @@ Pick the tag that matches the urgency so anyone scanning the code can tell a rel
 The [core data structures](core-data-structures/core.md) docs paste real type definitions so a reader sees the exact shape. To keep a paste from drifting when source changes, fence it as ` ```ts type-equiv ` (instead of ` ```ts `) and register it in `scripts/type-equiv.manifest.json` with the source file and symbol it mirrors:
 
 ```json
-{ "doc": "docs/core-data-structures/session.md", "symbol": "SessionEvent", "source": "packages/session/src/types.ts" }
+{ "doc": "docs/core-data-structures/session.md", "symbol": "SessionEvent", "source": "packages/core/session/src/types.ts" }
 ```
 
 `pnpm run verify-type-equiv` (part of `doc-sync`) then extracts that symbol's declaration from source via the TypeScript parser and asserts the block matches it (whitespace- and comment-insensitive, so a doc block may show a clean definition and the prose can carry the semantics). It also enforces a 1:1 correspondence: every `ts type-equiv` block has exactly one manifest entry and vice-versa, so a block can't go silently unchecked and a stale entry can't linger. `doc-typecheck` skips `ts type-equiv` blocks (they aren't standalone-compilable) and excludes them from its opt-out ratio. When you change a documented type, the gate fails until you update the paste; when you add or remove a block, update the manifest in the same change.