[Performance] Combine AsyncImageView optimizations

[NachoSoto]

Summary

This branch combines the independently reviewed changes from:

• [Performance] Skip unchanged UIKit image requests #77 — skip unchanged UIKit image sizes
• [Performance] Streamline SwiftUI size observation #78 — streamline SwiftUI geometry observation
• [Fix] Preserve disk-cached image scale #79 — preserve UIImage.scale in the disk cache
• [Performance] Remove redundant image scheduler hop #80 — remove the redundant image-scheduler hop
• [Performance] Lazily initialize SwiftUI image state #81 — lazily initialize SwiftUI image state
• [Performance] Reduce multicast renderer contention #82 — reduce multicast contention while preserving subscription semantics

Component benchmarks

Change	WatchChess workload	Result
UIKit unchanged-size guard (#77)	128,000 origin-only mutations	-32.24% mean, scheduler calls 128,000 → 0
SwiftUI geometry (#78)	100 real cells × 80 relayouts	-2.86% mean, 95% CI for saving 9.053–45.532 ms
Scheduler hop (#80)	96 cold + 96 repeated requests	scheduler operations -50%; wall time neutral
Lazy SwiftUI state (#81)	20,000 body reconstructions	-4.44% mean, 12/12 paired wins
Multicast completed hits (#82)	50,000 real PieceView hits	-89.3% / 9.33× faster
Multicast distinct keys (#82)	24 concurrent keys	neutral: -1.4% mean, 95% CI spans zero

Combined WatchChess benchmark

Release WatchChess, iPhone 17 Pro Max / iOS 26.5 simulators, 10 samples per variant and flow. Negative CPU/hitch changes are improvements.

CPU

Flow	CPU time, baseline → combined	Change (paired-bootstrap 95% CI)	Mean one-core-equivalent CPU
Tournament-list scrolling	2.230 → 2.264 s	+1.54% (-2.79% to +6.09%): neutral	12.01% → 12.07%
Rounds scrolling	1.059 → 1.024 s	-3.25% (-5.55% to -1.27%)	13.03% → 12.79%
Open game	0.452 → 0.439 s	-2.89% (-5.36% to -0.12%)	13.47% → 13.11% (-2.63%; CI -4.37% to -0.87%)

FPS and hitches

FPS below is a main-run-loop CADisplayLink callback proxy over the exact UI-action window, not compositor presentation FPS.

Flow	FPS, baseline → combined	FPS change (95% CI)	Hitches/min, baseline → combined	Hitch change (95% CI)
Tournament-list scrolling	58.93 → 59.00	+0.12% (-0.20% to +0.48%): neutral	40.67 → 37.39	-8.06% (-29.75% to +14.24%): neutral
Rounds scrolling	58.53 → 58.98	+0.77% (+0.13% to +1.39%)	63.24 → 51.68	-18.28% (-35.36% to +5.33%): trend, CI spans zero
Open game	54.14 → 54.07	-0.14% (-1.20% to +0.91%): neutral	131.04 → 145.44	+10.99% (-5.26% to +29.68%): neutral

Missed-refresh rates were also neutral for tournaments (-3.62%, CI -21.79% to +14.17%) and game (+2.99%, CI -5.93% to +13.06%). Rounds trended lower (-26.69%, CI -47.25% to +4.29%).

Methodology and controls

• two-pass, pass-outer crossover: pass 1 baseline on simulator A then candidate on B; pass 2 candidate on A then baseline on B
• five measurements per pass, variant, and flow; 10 total per variant/flow
• engine analysis forced off (currentEngineStrength = 1) and validated inside every app launch/report
• exact 24-entry real API fixture in cache-only mode; any cache miss failed the run
• identical 147-path warm AsyncImageView cache coverage per variant
• exact AsyncImageView path/hash/mtime validation before sample 1 and after every five-sample block
• API path/size/hash validation before and after every block
• arithmetic-mean CPU headline; duration-weighted ratio-of-sums FPS/hitch rates
• deterministic 100,000-resample, pass-stratified paired bootstrap confidence intervals
• symmetric full cache hashing before each measured block; these are deliberately warm-cache measurements
• hitches: clipped display-link interval at least 1.5× its target refresh period

All earlier measurements with uncontrolled engine state or unequal cache warmness were quarantined and excluded.

Rendering and cache regression checks

• cold game and rounds screenshots were pixel-identical; cold tournament app content was pixel-identical outside dynamic system chrome
• all four pass-1 seed/verify tournament screenshots were pixel-identical across the 1320×2590 app region; the only difference was 5,708 pixels in the translucent system search-field blur
• six gated 8 FPS recordings covered tournament scrolling, rounds scrolling, and opening a game for both variants
• manual contact-sheet review found no blank flashes, stale substitutions, missing portraits, or persistent placeholders; both game recordings showed the same normal transition placeholders and matching fully rendered end states
• all six recordings passed pre/post AsyncImageView and API cache audits
• baseline and candidate kept identical 147 relative cache paths; each variant's hashes and mtimes remained stable

The different cache bytes between variants are intentional: #79 stores image scale in a versioned envelope. Legacy raw-PNG entries become one normal miss and regenerate in the corrected format. Existing remote images may therefore re-download once; offline users can temporarily miss legacy cached images until connectivity returns.

#79 and #82 semantics

#79 and the #82 synchronous-subscriber review finding are different fixes, and both are included:

• [Fix] Preserve disk-cached image scale #79 fixes PNG disk-cache scale metadata.
• [Performance] Reduce multicast renderer contention #82 attaches the first subscriber before starting synchronous upstream work, preserving first-request miss / repeat-request hit semantics.

#82's lifetime.observeEnded intentionally holds the wrapper and in-flight entry only until producer completion or cancellation. Weak-reference tests verify release after both paths, while cancellation still disposes upstream work; no retain cycle or leak was observed.

Validation

• swiftlint --config .swiftlint.yml --strict: 0 violations
• full iOS simulator suite: 69 XCTest/Quick + 13 Swift Testing tests = 82 passed
• focused multicast concurrency, synchronous, retry, memory-warning, cancellation, and weak-lifetime coverage
• WatchChess Release cache preflights, 60 CPU samples, 60 frame reports, and six visual recordings passed their evidence checks
• CI updated to macOS 26 / Xcode 26.5