Enable bounded bbtree mmap dictionaries by default for 0.2.5862 #772

Problem

Warm mmap word dictionaries currently binary-search variable-length on-disk records. Paired local measurements across three source corpora show that a bbtree-style inline-key search can reduce this lookup cost without changing postings or retrieval results.

Proposed release work

• Enable the Zig read-only bbtree dictionary by default for mmap word indexes.
• Use a single exact-size allocation capped at 8 MiB; retain complete binary search on budget/allocation failure.
• Verify exact/prefix parity, promote-on-write cleanup, native CLI peak RSS, and MCP behavior.
• Release on current main, preserving the isolated watcher CPU fixes already integrated through fix: bound macOS watcher audits without retrieval changes (#756) #758/release: 0.2.5861 bound macOS watcher idle work #771 rather than merging the unrelated changes in fix: restore default semantic fallback and reduce macOS watcher work #750.

No ranking, candidate-pool, hosted-model, or disk-format change is intended.