Make <Syntax> cancellation always release its reference #775

Story

When a running <Syntax> reference is cancelled, its scoped cleanup always finishes before the test observes teardown, so cancellation evidence is deterministic.

Current gap

The existing SYN22: a cancelled reference tears down and commits no record case failed once at exact local revision 33a45a31558765ed9a647bd4a5add4c6c6b200e9, then passed without any source or test change. This is an intermittent lifecycle failure; the cause is not yet established.

The failure occurred during local feedback review, so there is no CI run link.

Evidence

Runtime:

deno 2.9.5 (stable, release, aarch64-apple-darwin)
v8 15.0.245.2-rusty
typescript 6.0.3

Failing command:

deno task test packages/core/tests/generated-xmd.test.ts packages/core/tests/evaluate-component.test.ts packages/core/tests/syntax-component.test.ts packages/core/tests/evaluation-profile.test.ts packages/core/tests/evaluate-provider-lifetime.test.ts packages/core/tests/evaluate-loaded-copy.test.ts packages/core/tests/syntax-loaded-copy.test.ts

First failure output:

Tier SYN — the record one occurrence keeps ... SYN22: a cancelled reference tears down and commits no record

- Expected  - 3
+ Received  + 1

- Array [
-   "released",
- ]
+ Array []
    expect(teardown).toEqual(["released"]);

FAILED | 40 passed (314 steps) | 1 failed (1 step) (6s)

Passing rerun, with no intervening change:

deno task test packages/core/tests/syntax-component.test.ts --filter 'record one occurrence keeps'

SYN22: a cancelled reference tears down and commits no record ... ok (22ms)
ok | 1 passed (4 steps) | 0 failed | 6 filtered out (194ms)

Acceptance

• The cancellation case reliably observes reference cleanup before completing.
• Focused lifecycle evidence rejects an implementation that merely makes the timing window less frequent.
• The cause and regression evidence are recorded before this issue is closed; a green rerun alone does not close it.