chore: migrate TODO.md to TODO.local.md per private-backlog directive (#140)

Per the global Claude Code directive:
- Public actionable backlog → GitHub Issues (only public surface)
- Private working memory → `TODO.local.md` (gitignored, never tracked)
- "No carve-outs" — the historical track-by-default compromise is retired

Concrete changes:
- Open Issue #139 for the sole remaining Pending-LOW item
  (engines.node bump consideration, deferred wait-and-see).
- Add `*.local.md` to `.gitignore` so future `TODO.local.md` files
  (and any other `.local.md` working notes) are never tracked.
- `git rm TODO.md` — the file leaves the public repo. The maintainer
  keeps a `TODO.local.md` locally as private working memory; new
  contributors pulling the repo will see only GitHub Issues as the
  authoritative backlog source.

CLAUDE.md was already updated in PR #138 to point at GitHub Issues
and to drop the `TODO.md` entry from the project structure tree;
this PR completes the migration on the file-system side.

Author: oorabona

.gitignore

@@ -121,4 +121,8 @@ build/wasm/
 
 # TypeDoc output & demo build
 docs-site/
-examples/browser/node_modules/
+examples/browser/node_modules/
+
+# Maintainer working memory — per global Claude Code directive
+# (public actionable backlog → GitHub Issues; *.local.md files are never tracked)
+*.local.md