CI: hatterSrc = ../. invalidates nix cache on any file change

[jappeace-sloth]

Problem

The watchOS CI job timed out (6h) on PR #207, and android had a SIGSEGV crash. Both are likely caused by nix cache invalidation.

Root cause

In nix/watchos.nix (and ios.nix, android.nix):

hatterSrc = ../.;

This passes the entire repo as the source. When ANY tracked file changes (e.g. watchos/Hatter/HaskellBridge.swift), the nix source hash changes, which invalidates ALL derivations that depend on hatterSrc — including the GHC cross-compilation step.

On master, the derivation hashes match the community cache, so everything downloads pre-built. On a PR branch with even a one-line Swift change, the entire GHC cross-compile must run from scratch on the CI runner.

Impact

• watchOS: 6-hour timeout — the macOS runner couldn't finish the full cross-compile in time
• android: The SIGSEGV may be related to a cache-miss rebuild producing a subtly different binary (different optimization, linking order, etc), or it may be genuinely flaky
• iOS: Passed because its runner was fast enough (30 min)

Possible fix

Split hatterSrc so the GHC build derivation only depends on src/, cbits/, include/, and the cabal file — not Swift files, project.yml, or other platform files. The Swift files could be passed as a separate input to the installPhase. This way, changing a Swift file wouldn't invalidate the expensive GHC cross-compilation cache.

Context

• Master CI: consistently green, 30-37 min (everything cached)
• PR Limit GHC RTS heap on iOS/watchOS #207 CI: watchOS timeout, android flaky (cache miss → full rebuild)
• PR Limit GHC RTS heap on iOS/watchOS #207 only changes ios/Hatter/HaskellBridge.swift and watchos/Hatter/HaskellBridge.swift

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