feat: add experimental greedy prepare_context (not C3)

[guozhihao-224]

Summary

• Pack Memory FTS hits in @powercontext/builtin with a greedy UTF-8 budget (newline join, no mid-entry truncation) and expose ExperimentalRuntime.prepare().
• Register POST /v1/context/prepare on the experimental subset Server and advertise powercontext.prepared-context.v1 in capabilities.
• Empty vs ready is honest: no matches or an oversized-only hit returns status: empty, content: null; max_bytes below 512 is 422.
  This is not Python PreparedContext packing parity and not C3.

Profile and parity

• Profile: experimental sqlite-fts subset (not a sqlite-fts / C3 claim)
• Target level: C0 packing + C1 wire for prepare_context only
• Related issue or RFC: Memory, FTS, and PreparedContext #4 (narrowed; experimental packer on TS-created DBs). ADR 0008
• Does not close SQLite kernel and schema-version gate #3 or Memory, FTS, and PreparedContext #4

Checklist

• Does not add a second wire contract
• Does not hand-edit contract/ or conformance/ snapshots
• Does not hand-edit packages/protocol/src/generated
• Baseline bump, if any, is this pull request’s only purpose (no bump)
• New or changed behavior names the compatibility decision or ADR

Test plan

• pnpm format:write
• pnpm lint
• pnpm typecheck
• pnpm build
• pnpm test
• pnpm generate:check
• CJK remember → prepare_context is ready and content_bytes matches utf8ByteLength
• no hits → empty / content: null
• 513-byte hit with max_bytes: 512 → empty (no truncation)
• max_bytes: 511 → 422
• capture still 503; user README still says HTTP Server not shipped