Add SwiftLint, pre-commit hooks, and CI workflow

[silentswordfish]

Summary

• SwiftLint (.swiftlint.yml): opts into a handful of stricter rules (force unwrapping, implicitly unwrapped optionals, empty_count, etc.), caps line length at 120/200 (warning/error), and whitelists the small set of short/underscored identifiers already used in this codebase (s, t, fm, op, and the vXX_Y version-literal test names) rather than loosening those checks globally.
• pre-commit (.pre-commit-config.yaml): a local hook that runs swiftlint lint against staged Swift files via the pre-commit framework. Documented in the README under Development.
• CI (.github/workflows/ci.yml): two parallel macOS jobs on push/PR — build+test (Xcode pinned via setup-xcode, SPM build cache) and lint (swiftlint lint).
• Ran swiftlint --fix for the mechanical violations this surfaced (trailing commas, trailing newlines, one colon-spacing fix) across existing files, plus one count > 0 → !isEmpty cleanup. Everything else that remains is an advisory warning, not blocking — pre-existing structural patterns (function length/param count, an implicitly-unwrapped optional in the FSEvents callback context) are left for their own future PRs rather than a drive-by rewrite here.

Test plan

• swift build and swift test pass (40 tests)
• swiftlint lint exits 0
• pre-commit install + smoke-tested that the hook actually blocks a commit with a real violation, then verified a clean commit passes
• Confirm the ci.yml workflow runs green on GitHub (first run once this PR is opened)

🤖 Generated with Claude Code

[butterbearbear]

hashFiles('Package.resolved') always hashes an empty match here because Package.resolved is not committed, so the cache key stays the same across every PR even when Package.swift or the floating branch: main deps drift. Caching the whole .build tree on top of that can also restore stale build products across PRs or toolchain changes. Would it make sense to key off Package.swift (or commit Package.resolved) and cache SwiftPM’s package caches instead of the full .build directory?

[silentswordfish]

Good catch. Fixed: keyed off Package.swift instead, and narrowed the cached paths to ~/Library/Caches/org.swift.swiftpm + .build/checkouts + .build/repositories (fetched sources only) rather than the whole .build tree, so a restore just saves the floating-branch deps an incremental git fetch instead of a full clone — it can no longer serve stale compiled objects across dependency or toolchain drift.

[butterbearbear]

SwiftLint’s default trailing_comma rule forbids trailing commas (mandatory_comma: false), which is why autocorrect dropped them on multiline arrays like this. A lot of Swift projects flip that to mandatory_comma: true so list edits stay quieter in diffs. Do we want that for this repo?

[silentswordfish]

Agreed, flipped to mandatory_comma: true and re-ran autocorrect to put the trailing commas back.

[butterbearbear]

brew install swiftlint tracks Homebrew’s latest formula, so an upstream rule change can fail this job with no repo diff. Want to pin a SwiftLint version here so the lint gate stays reproducible?

[silentswordfish]

Pinned to 0.63.3 (same version installed locally) via the portable_swiftlint.zip release asset instead of brew install, so the lint gate can't go red from an upstream rule change with no diff in this repo.