ADR 0025: Revision-bound line review threads

Status: implemented in development after beta.4.

Plain discussion comments and quoted code do not represent reply groups or a resolution state. Add separate repository-local review threads, without using approval records for discussion. Existing comments and review votes remain intact.

Each thread stores the proposal, immutable base/head IDs, path, old/new side, line number, server-read line text, initial body and author. Creating a thread requires the current proposal head and a valid text line in the selected revision. The server reads the line itself; client-supplied text is never authoritative. The old side requires a base revision. Unsupported/binary or oversized source files fail explicitly. A thread's revisions remain GC roots, including after resolution, proposal closure or a proposal-head replacement. Integrity checks report an unreadable source as dangling_thread_revision.

Replies are immutable and independently paged. They increment the thread version. Resolution and reopening require the expected thread version, so a concurrent reply cannot be silently marked resolved. Thread authors, proposal authors, reviewers and integrators can change resolution; other writers can discuss. Resolved threads must be reopened before adding a reply. All writes use the existing repository authorization, audit/outbox and transaction-scoped idempotency. Discussion state does not itself approve or block integration in this slice.

Metadata schema 6 adds review_threads and review_thread_comments to SQLite and PostgreSQL. Existing rows are unchanged. Upgrades are transactional and require a pre-upgrade backup. Older binaries refuse schema 6; rollback restores the backup with the old binary instead of opening upgraded storage. The deterministic VCS object format and merge semantics do not change.

Expose paged HTTP and gRPC operations, generated SDKs and the proposals.threads capability. The browser preserves its legacy comment composer on older servers. Show anchor revision explicitly and label threads on previous proposal heads as outdated, never silently remap line numbers. Suggestions, reviewer assignment and policy requirements for resolution remain separate work.