audio: don't reset a healthy pipeline on tab refocus

[pcarrier]

Problem

Refocusing the blit tab caused a brief audio dropout. The visibilitychange handler in AudioPlayer unconditionally called resetPipeline() whenever the tab became visible after being hidden for more than 3 seconds — tearing down the AudioContext, decoder, and worklet, then rebuilding on the next frame (async worklet reload + decoder reinit + re-buffering to bufferTarget).

The reset was meant as preemptive recovery, but browsers exempt audibly-playing tabs from background throttling: the pipeline usually keeps rendering fine while hidden, so the reset was interrupting working audio on every refocus.

Fix

Gate the reset on evidence of actual breakage:

• worklet position reports (posted every ~100 ms while the context runs) stale for >3 s, or
• the AudioContext is "closed".

Otherwise run an immediate checkHealth(), which still resumes a suspended context and escalates to a reset on real stalls — so recovery for genuinely broken background audio is preserved.

Testing

• tsc --noEmit clean, all 566 js/core tests pass.
• Manual repro: play audio, background the tab >5 s, refocus — no dropout; audio continues seamlessly.

[indent]

PR Summary

Stops AudioPlayer from tearing down and rebuilding a healthy audio pipeline every time the blit tab is refocused. The old visibilitychange handler reset unconditionally after the tab was hidden >3 s, causing a ~100-200 ms audible dropout even though browsers keep audibly-playing tabs rendering while hidden. The reset is now gated on real breakage evidence, otherwise it defers to the existing checkHealth() path.

• Refocus reset condition tightened from wasHiddenMs > 3_000 to wasHiddenMs > 3_000 && (workletStale || ctxDead), where workletStale = worklet position reports (posted ~every 100 ms) gone stale >3 s and ctxDead = AudioContext.state === "closed".
• All other refocus cases now run an immediate checkHealth(), which still resumes suspended contexts and escalates to a reset on genuine worklet/decoder stalls, so recovery for actually-broken background audio is preserved.
• Comment rewritten to document the audible-exempt rationale; change is confined to the startHealthCheck() handler with no API, wire, or consumer impact.

Issues

Review closed.

CI Checks

All CI checks passed on c0a2474.

_{View session}

[github-actions]

Coverage

Crate	Lines	Functions	Regions
alacritty-driver	69.7% (698/1002)	72.0% (54/75)	71.7% (1050/1464)
browser	0.0% (0/807)	0.0% (0/65)	0.0% (0/1370)
cli	17.2% (1521/8821)	28.1% (200/712)	20.1% (2730/13555)
compositor	2.6% (246/9558)	7.3% (32/439)	2.7% (352/12801)
fonts	81.4% (721/886)	88.6% (70/79)	83.0% (1427/1719)
fssync	92.5% (4880/5274)	94.1% (445/473)	92.5% (8926/9647)
gateway	26.1% (375/1437)	29.9% (38/127)	19.9% (470/2360)
git	87.3% (4165/4771)	88.2% (328/372)	87.0% (6594/7582)
lsp	76.0% (2503/3295)	78.2% (248/317)	73.8% (3886/5266)
proxy	19.3% (172/892)	20.5% (26/127)	21.2% (293/1381)
remote	90.2% (8628/9561)	92.9% (619/666)	88.0% (14359/16310)
sd-notify	73.9% (68/92)	100.0% (6/6)	83.2% (109/131)
server	37.3% (6504/17458)	51.0% (624/1224)	39.9% (11112/27824)
ssh	1.9% (7/374)	3.2% (1/31)	0.7% (4/613)
upsidedown	31.4% (391/1247)	27.8% (55/198)	34.8% (797/2287)
webrtc-forwarder	2.7% (72/2624)	2.1% (4/187)	1.2% (50/4335)
webserver	60.3% (1051/1742)	64.7% (156/241)	63.1% (1800/2851)
Total	45.8% (32002/69841)	54.4% (2906/5339)	48.4% (53959/111496)

[github-actions]

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