feat(rpc): persistent bounded index pages and compaction for the compact consumer (S03b-1b-I-b, part of #4272)

changes/unreleased/4386.md

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+<!-- changes-log: packages/coding-agent/src/modes/rpc/changes.md -->
+## 2026-10-26 - Wire persistent bounded index pages (S03b-1b-I-b, part of #4272)
+
+### What changed
+
+- `packages/coding-agent/src/modes/rpc/compact-content-index.ts` uses counted persistent roots, bounded descriptor pages, and operation leases instead of copying the complete base and overlay at freeze.
+- `packages/coding-agent/src/modes/rpc/compact-index-tree.ts`, `compact-index-compactor.ts`, and `compact-index-pages.ts` implement changed-path publication, bounded compaction, stale-row ownership checks, and strict row/byte packing limits.
+- `packages/coding-agent/src/modes/rpc/compact-read-lease.ts` rechecks owner validity when an operation completes, including retirement during an in-memory read.
+- `packages/coding-agent/src/modes/rpc/compact-session-state.ts` uses the new frozen index through the real settled-message reader. The codec staging guard is removed with this production wiring.
+- Index traversal reuses a decoded page across its rows; compatibility assembly lives in `compact-index-traversal.ts`. Compaction builds a private persistent root before publication and reclaims only unpublished pages on failure. Index writes release partial codec owners when typed limits refuse a key or patch.
+
+### Why
+
+- Freeze and publication must not enumerate the entire collection. Oversized rows and keys need references rather than oversized page files, and older compactions must not erase newer state.
+
+### Why an extension could not handle it
+
+- The host's synchronous current-state owner and asynchronous storage lifetime sit below extension delivery.
+
+### Expected merge conflict zones
+
+- Compact settled-index ownership and frozen-root types. Exact resident membership remains until S03b-1b-II; active component migration and RULING-33 are not part of this increment.