refactor(tool-fs): consolidate read rendering; drop the fs/observed try-catch

Two cohesion cleanups on the filesystem tool package:

- Fold window.ts + types.ts + formatReadOutput into one cordis-free
  read-render.ts. Line windowing, the FileReadOutcome shape, and output
  formatting are one concern (the read tool's rendering); splitting them across
  three files added no value. read.ts is now just the tool (schema + I/O).

- Drop observe.ts and emit fs/observed with a plain ctx.emit in read/write/edit.
  The event is contractually a synchronous, side-effect-only recorder
  (file-context's listener is a WeakMap.set), so the per-call try/catch guarded
  against a contract violation that cannot happen under the shipped listener —
  defensive code for an impossible case. The event contract (dsh-fs JSDoc,
  README, RFC) is updated to state the fire-and-forget semantics plainly.

scripts/type-equiv.manifest.json

@@ -51,7 +51,7 @@
     { "doc": "docs/core-data-structures/filesystem.md", "symbol": "FsEditOutcome", "source": "packages/fs/fs/src/types.ts" },
     { "doc": "docs/core-data-structures/filesystem.md", "symbol": "FsErrorCode", "source": "packages/fs/fs/src/types.ts" },
     { "doc": "docs/core-data-structures/filesystem.md", "symbol": "FileContextExec", "source": "packages/fs/file-context/src/types.ts" },
-    { "doc": "docs/core-data-structures/filesystem.md", "symbol": "FileReadOutcome", "source": "packages/fs/tool-fs/src/types.ts" },
+    { "doc": "docs/core-data-structures/filesystem.md", "symbol": "FileReadOutcome", "source": "packages/fs/tool-fs/src/read-render.ts" },
 
     { "doc": "docs/core-data-structures/compaction.md", "symbol": "CompactionResult", "source": "packages/compact/compact/src/types.ts" },