fix(state): keep token registries across a root reconnect (#273) - #277

Closes #273.

What was wrong

Re-attaching the root <wcs-state> — host.remove(); document.body.appendChild(host), or any move of it in the DOM — silently stopped $on handlers, on scans and command-token subscriptions. There was no error and no warning.

• disconnectedCallback threw away the command-token and event-token registries.
• $on (and an on scan) subscribes only in the _state setter, which a reconnect does not run.
• A command.<method>: binding subscribes only when its value is applied, which a reconnect does not do either.
• The firing side resolves the token from the registry every time, so after reconnecting it created a fresh token with no subscribers.

Fix

• Disconnecting keeps both registries, the same two-step shape $streams and $watch already use (stop firing, keep the registry).
• Nothing fires while the element is disconnected. An element event finds no state tree (No state tree found on this root for eventToken handler), and $command cannot be read because the state cannot be created (State rootNode is not available.). Both are pinned in tests.
• A re-set still replaces the $on subscriptions.
• clearCommandTokenRegistry lost its only caller and is removed.

Tests

• New integration.tokenReconnect.test.ts goes from real binding strings to dispatch and emit. It covers $on after a reconnect, repeated reconnects (still one subscription), a re-set after a reconnect, events while disconnected, command.<method>: with $command emit, onclick: $command.<name>, and emit while disconnected.
• The DEFECT(#273) test in scan.on.test.ts is flipped.
• Before the fix, 4 of the 5 new reconnect tests failed, along with the flipped scan test. The re-set test passed, as it should.

Docs

• The README (en/ja) no longer says the registries are cleared on disconnect.
• The "known gap" notes for on scans are removed from the README, docs/scan(.ja).md and CHANGELOG.md.
• docs/state-scan-design.md D8 and §5-3 record the fix.

Verification

e2e runs in CI.

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