feat: add abortErrorGuard reusable

[Guria]

Adds abortErrorGuard — a test-time collector for unexpected AbortErrors surfaced on .onReject lifecycle actions.

• installAbortErrorGuard() registers a global extension once (idempotent) that hooks every *.onReject action and records payloads that are aborts (isAbort). Returns { clear, drain, format }; also exported as named clearAbortErrors / drainAbortErrors / formatAbortErrors.
• drain() groups identical errors by action name + message with a count, then empties the buffer.
• Intended for a Storybook/Vitest beforeEach teardown that fails a test when aborts leaked during it.
• Extracted from a production app (guria/modern-stack .storybook/abortErrorGuard.ts); the app-specific regression assertion was dropped and documented as a recipe instead.

Docs note the caveat that addGlobalExtension cannot be uninstalled (process-wide, permanent); clear/drain reset only the buffer. Node tests cover collection, grouping/count, non-abort exclusion, drain/clear, and single-registration.

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