fix(store): guard abort on request supersession

[mihar-22]

Summary

• Add abortable utility to race promises against abort signals
• Wrap queue handlers and store guards with abortable to ensure immediate rejection on supersession
• Fix hanging guards when requests are rapidly superseded (e.g., seek scrubbing)

Problem

Guards and handlers may not respect the abort signal. If a request is superseded while waiting in a guard (e.g., waiting for canplay), the guard promise could hang forever:

store.request.seek(10);  // waiting in guard for HAVE_METADATA...
store.request.seek(20);  // supersedes first request
// First request's guard was still waiting, holding references

Solution

The abortable(promise, signal) utility races any promise against an abort signal, guaranteeing immediate rejection regardless of whether the guard/handler implementation checks the signal.

Behavior Change

Abort errors now carry the original reason (SUPERSEDED, ABORTED) rather than always being ABORTED. This provides more accurate error information—callers can now distinguish "user cancelled" from "superseded by newer request."

Test Plan

• 7 new tests for abortable utility
• All 257 store tests pass
• All 154 utils tests pass

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4 Skipped Deployments

Project	Deployment	Review	Updated (UTC)
vjs-10-demo-html	Ignored		Jan 19, 2026 2:56pm
vjs-10-demo-next	Ignored		Jan 19, 2026 2:56pm
vjs-10-demo-react	Ignored		Jan 19, 2026 2:56pm
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