chore: add 'mmap' to cspell dictionary

[bdero]

Summary

Unblocks CSpell on main. The 1.6.98 entry in RELEASE_NOTES.md references mmap (the Unix syscall) when describing the Android updater change to mmap libapp.so out of the APK; cspell's full-repo scan in .github/workflows/main.yaml flags the word and the CSpell GitHub-Actions check has been failing on main for every push since that release note landed.

What this changes

One line: adds mmap to the alphabetical word list in cspell.config.yaml, between mktemp and mocktail.

   - mktemp
+  - mmap # Unix `mmap` syscall — referenced in RELEASE_NOTES (libapp.so mmap-from-APK)
   - mocktail

Why the global config and not inline-scoping

RELEASE_NOTES.md already has an inline <!-- cspell:words ... --> directive at the top, so inline-scoping was an option. I went with the global config because:

• mmap is a generic Unix term (sits naturally next to mktemp in the existing list).
• The Rust updater's mmap work is likely to surface in future Dart code, comments, log messages, or release notes — adding it once globally avoids whack-a-mole later.
• Other syscalls / low-level terms (dyld, dylib, mktemp) live in the global config too; this matches the convention.

Verification

gh api repos/shorebirdtech/shorebird/check-runs/<id>/annotations for the most recent failing CSpell run on main reported a single annotation:

RELEASE_NOTES.md:13:14 — Unknown word (mmap)

That annotation is the only failure surfaced by the check, so this should fully unblock CSpell on main. Discovered while debugging cspell on #3750 — that PR's incremental-only check (GitHub Actions) is green; the full-repo CSpell (Shorebird-internal CI) inherits this main failure.

Test plan

• Annotation contents inspected on the latest failing CSpell run for main.
• Word added in alphabetical position next to existing similar entries.
• CI: CSpell now green on this PR (will land on push).