feat(store): expose mmap_size via CBM_SQLITE_MMAP_SIZE env

Hard-coded `PRAGMA mmap_size = 67108864` in configure_pragmas() left
no path for users running multiple cbm-mcp instances against the
same cache to opt out of memory-mapped I/O. On macOS, when one
instance's checkpoint or reindex truncates the DB file under
another instance's live mmap, accessing the now-missing pages
raises SIGBUS, taking the process down.

Setting CBM_SQLITE_MMAP_SIZE=0 reverts to read()/pread() I/O,
which returns recoverable SQLITE_IOERR instead of crashing the
process.

- Default unchanged (67108864 / 64 MB). No behavior change for
  single-instance users.
- Malformed values (non-numeric, partial-numeric) fall back to
  the default rather than failing the store open.
- Negative values clamp to 0.
- New tests: tests/test_store_pragmas.c covers all five resolver
  paths plus an integration smoke that opens a file-backed store
  with mmap disabled.

Empirical evidence: 9 SIGBUS crash reports collected on macOS
arm64 v0.6.0 in a 14-hour window, all signature 'cluster_pagein
past EOF' with stacks bottoming in SQLite btree code under the
watcher thread's incremental-index pipeline.