fix: auto-index mid-size exec output for ctx_search without intent

[niemst]

What / Why / How

ctx_execute / ctx_execute_file output between 5 KB and 100 KB was returned raw inline but never indexed — the gap between the 5 KB intent threshold and the 100 KB pointer threshold — so a later ctx_search could not find it. Index that range into the FTS5 content KB as a side effect at the 5 KB threshold while still returning stdout inline; the intent search and >100 KB pointer paths are unchanged. Each exec gets a distinct source label (execute:<lang>:<hash> / file:<path>:<hash>) so successive execs don't clobber each other's index.

Affected platforms

• All platforms

(core exec/index path in src/server.ts — platform-agnostic)

Test plan

Added a regression test in tests/core/server.test.ts: a ~6 KB no-intent ctx_execute returns raw inline and is retrievable via search. npm test green (one pre-existing macOS-locale test unrelated to this change), npm run typecheck clean, npm run build passes (tsc + bundle + assert-bundle + assert-asymmetric-drift).

Checklist

• Tests added/updated (TDD: red → green)
• npm test passes
• npm run typecheck passes
• Docs updated if needed — n/a
• No Windows path regressions (forward slashes only)
• Targets next branch