perf(ios): cache ISO-8601 parsers instead of allocating per call

[BunsDev]

Summary

Fourth PR in the iOS performance pass. Cuts avoidable allocations in list scrolling/sorting.

Root cause

caveParseISO() allocated up to two ISO8601DateFormatter instances on every call, and it's called per item inside sorted/filter/map across the tasks, calendar, reminders, and thread lists (e.g. FamiliarThreadsView sort comparator, CalendarView grouping). ISO8601DateFormatter is comparatively expensive to construct, so this was avoidable churn during scroll and re-sort.

Fix

Hoist the two formatters (fractional + plain) to shared, lazily-initialized module-level instances and reuse them. ISO8601DateFormatter is thread-safe for reads; parsing behavior is identical (try fractional, then plain).

Verification

• xcodebuild build (scheme CovenCave, iOS Simulator) → BUILD SUCCEEDED.

Blast radius

Tiny — 1 file, pure internal caching. Same inputs/outputs.

Notes

Part of the perf series (#3454 chat render, #3455 stream coalescing, #3456 persist debounce). CI does not build the Swift app; local xcodebuild is the iOS gate.

[Copilot]

Pull request overview

This PR improves iOS list scrolling/sorting performance by avoiding repeated ISO8601DateFormatter allocations during ISO-8601 parsing, which is used heavily in per-item sort/filter/map paths across multiple list views.

Changes:

• Hoists two ISO8601DateFormatter instances (fractional + plain) into shared cached instances.
• Updates caveParseISO(_:) to reuse the cached formatters instead of allocating new ones per call.

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[romgenie]

Verification complete

Reviewed the exact head 21f25a1fafe2916db1109ab6934d47990d108cc8. The cached formatters preserve the existing fractional-then-plain fallback order; no actionable defects or review threads were found, and current main is an ancestor of the PR head (no conflict). No follow-up commit was needed.

Validation:

• git diff --check origin/main...HEAD
• pnpm check:tests-wired — 1,072 test files wired
• Exact-head GitHub CI is green, including Frontend build (typecheck, app/API/mobile tests, production build), E2E (104 passed, 2 skipped), Rust, cross-environment, Linux/Windows sidecar runtime, CodeQL, and the required aggregate checks.

The local WSL worktree does not provide Xcode; the PR author's recorded iOS Simulator xcodebuild result remains the native iOS build evidence. The branch is MERGEABLE / CLEAN and ready for maintainer review.