fix(images): export light/dark pairs together with granular cache

[alexey1312]

Problem

ImagesLoader.loadFromSingleFileWithGranularCache filtered to changed nodes without pairing light/dark variants. A designer edit touching only the dark variant produced an orphan dark asset, and ImagesProcessor then rejected the whole config:

Error: Asset count mismatch: light=0, dark=1
Missing assets in Light: safety-sos-button-color

f124526 added pairing to IconsLoader only — ImagesLoader was never updated, and both of its branches were affected (the PNG branch had no pairing-aware helper at all). This broke 03_daily_assets_update in Oymyakon-Atoms-iOS on every day the illustrations file changed (4 runs in July).

Changes

• loadPNGImagesWithGranularCache takes darkModeSuffix: String? = nil and routes to fetchImageComponentsWithGranularCacheAndPairing when set
• single-file PNG branch passes darkSuffix; SVG branch switches to loadVectorImagesWithGranularCacheAndPairing
• loadFromLightAndDarkFileWithGranularCache keeps nil on purpose — light and dark live in separate files and are filtered independently

Also fixed (found during the same investigation)

• batch returned exit code 0 while reporting failures, so CI treated a broken export as green — now throws ExitCode.failure when failureCount > 0
• the failure table truncates both config name and error, making CI logs useless for diagnosis — full errors are now printed underneath it
• 5 tests in SubcommandFaultTolerancePrecedenceTests crashed on ParsableArguments.init(), which leaves every @Option undecoded so the first read traps. Switched to try FaultToleranceOptions.parse([]), the pattern already used throughout FaultToleranceOptionsTests. Under --parallel this surfaced as a non-zero exit with no XCTAssert failure line, which is why it read as a green suite.

Tests

New ImagesLoaderGranularCachePairingTests covers both branches:

• testOnlyDarkChanged_vectorBranch_includesBothVersions
• testOnlyDarkChanged_pngBranch_includesBothVersions

Full suite: 1996 XCTest + 211 swift-testing, exit 0 (swift test --parallel, Swift 6.3).

Docs

Updated .claude/rules/cache-granular.md, gotchas.md, and modification-checklists.md so the pairing requirement is recorded next to the granular-cache rules it belongs to.

[gemini-code-assist]

Caution

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