Stop a key getting stuck when Shift changes it while held (#227)

Input remembered keys under KeyboardEvent.key. Pressing a, adding
Shift and letting go sent "A" on release, which matched nothing, so
"a" stayed down until the next release without Shift.

- New rule keyName in src/core/keys.ts lowercases a single character.
  So a and A are the same key, also under Caps Lock.
- Input remembers per physical key (e.code) under which name it went
  down and releases exactly that name. So 1 released as ! is freed
  too, which lowercasing alone does not cover.
- On key down, up and held, Key is down and Axis input compare with
  the same rule. A node that already stores A reacts to a.
- Recording a key in the inspector stores Shift+A as a.
- Limit: holding Shift before pressing 1 still sends !, and a node on
  1 does not react. Nothing gets stuck by it.

Tests:
- 3 new unit tests in tests/core/behavior-events.test.ts: the flow
  from the issue, 1 released as !, and a node on A hearing a.
- Without lowercasing exactly the issue test and the A test fail.
- Without the code memory exactly the 1/! test fails.
- ./check.sh: 159 files, 2340 tests, CHECK EXIT 0 (before 2337).
- No e2e change, the check sum stays at 1377.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/core/keys.ts

@@ -0,0 +1,8 @@
+/**
+ * The name a key goes by in the graph: `KeyboardEvent.key`, with a single character lower-cased.
+ * Shift and Caps Lock turn "a" into "A" on the way down or up, but it is still the same key –
+ * without this, a key pressed as "a" and let go as "A" would stay held (#227).
+ */
+export function keyName(key: string): string {
+  return key.length === 1 ? key.toLowerCase() : key;
+}