Add dart_node_sql_js as a publishable package (sql.js bindings)

[MelbourneDeveloper]

TLDR

Promote dart_node_sql_js from a publish_to: none stub into a full tier-2 publishable package (typed sql.js / SQLite-on-WASM bindings) for the 0.13.0-beta release.

What Was Added?

• packages/dart_node_sql_js as a real publishable package: README.md, CHANGELOG.md (## 0.13.0-beta), LICENSE, analysis_options.yaml (austerity), dart_test.yaml, package.json (sql.js ^1.14.1).
• 32 integration tests in test/database_test.dart covering open/prepare/exec/run/get/all/save/close/pragma, data types, transactions and constraints — 100% line coverage (71/71).
• Wiring: tools/lib/packages.dart (tier 2, node), tools/test.sh (NODE_PACKAGES + TIER2), coverage-thresholds.json (dart_node_sql_js: 100.0), publish-tier1.yml changelog validation, publish-tier2.yml publish list, release skill.

What Was Changed or Deleted?

• Rewrote lib/src/database.dart and lib/src/statement.dart to use typed dart:js_interop extension types instead of nullable as casts, so the package passes dart analyze under austerity.
• Bug fix: sql.js export() frees every live prepared statement, so the previous "auto-save after each write" design broke prepared-statement reuse (Statement closed) and rolled-back transactions silently committed. Persistence now happens on close() / explicit save() only. Added save() to the Database API. Removed publish_to: none; set version 0.13.0-beta.

How Do The Automated Tests Prove It Works?

• Statement.run inserts multiple rows with incrementing rowid, updates rows and returns changes count, deletes rows and returns changes count — these failed before the fix (only the first write survived because export() closed the reused statement) and pass now.
• Transactions rolls back transaction asserts the balance is restored to 100 — previously reported 200 because the mid-transaction export persisted uncommitted state.
• save persists changes that survive reopen and loads an existing database file prove the new persistence model round-trips to disk.
• openDatabase returns error when the path cannot be read and Database.close returns error when the save target directory is missing cover the Result error paths. Full suite: 32/32 passing, 100% coverage (enforced by the ratchet in tools/test.sh).

Spec / Doc Changes

• New package README.md documents the async initializeSqlJs() + save/close persistence model. Release skill (SKILL.md) tier-2 lists updated to include dart_node_sql_js.

Breaking Changes

• None (new package; first publish at 0.13.0-beta)