fix: keep memory writes and indexing correct under Bun (#144700)

fix: keep memory writes and indexing correct under Bun (#144700)

Sort memory files before source-wide embedding batches so indexing order is stable across runtimes. Preserve POSIX symlink and parent-traversal semantics for Bun promotion writes, including dangling targets, lexical collisions, and non-directory traversal.

Adapt the SQLite context and worker-stderr tests to exercise the same invariants under Node and Bun. Exact-head proof passed 107 focused SQLite cases, 363 SQLite infra cases, and 1,542 memory-core cases under the custom Bun runtime; Node checks, typechecks, lint, formatting, and independent P0-P2 review also passed.