refactor(core): decouple snapshot type from engine (v0.3.2)

[eagle-head]

Description

Structural, internal-only refactor of @timekeeper-countdown/core, plus the lockstep v0.3.2 patch release of both packages. No public API or behavior change — this is a PATCH (verified by a release-readiness audit: the moved type stays barrel-exported with an identical shape, and the only touched behavior is an unreachable, non-exported internal guard).

• Decouple the snapshot type from the engine — move the CountdownSnapshot interface out of api/countdown-engine.ts into a dedicated leaf module src/model/countdown-snapshot.ts, removing the format/ → api/ layer inversion. It is still re-exported from the package root barrel with a byte-identical six-field shape, so consumers importing it from @timekeeper-countdown/core are unaffected.
• Consolidate the time-validity guard — collapse the three repeated guards in runtime/time-providers.ts into one module-private isValidTime(), restoring the MAX_SAFE_INTEGER upper bound the fallback path had dropped. Internal only (createSafeTimeProvider is not exported) and unreachable for a real Date.now().
• Single source of truth for the snapshot test double — the published testing-utils buildSnapshot now delegates to the engine's canonical buildSnapshot, so it can no longer drift from production (same parts / isRunning / isCompleted rules). Signature and output unchanged.
• Tests — add property and contract suites (time-provider coercion boundaries; snapshot-double vs. engine equivalence) and expand the per-unit mutation suites. Core: 453 tests.
• Docs — document the new src/model and src/time/clamp modules in CLAUDE.md; clarify in CONTRIBUTING that the Node 22 prerequisite is the build/dev toolchain, while the published packages support Node ≥ 18 at runtime.

Type of Change

• Refactoring (no functional changes)
• Chore (dependencies, CI, build, etc.) — lockstep v0.3.2 version bump

How to Test

1. npm install
2. bin/quality-gate.sh — full gate (build + format:check + lint + typecheck + test) is green: core 453 tests, react 18.
3. Confirm the public type still resolves from the barrel: import type { CountdownSnapshot } from '@timekeeper-countdown/core' (type-only, unchanged shape).

Checklist

• My code follows the project's code style
• I have added/updated tests for my changes
• npm run lint passes
• npm run typecheck passes
• npm run test passes
• I have followed the commit convention
• I have updated the CHANGELOG (packages/*/CHANGELOG.md) — recorded as an internal-only note for the 0.3.2 release (not a user-facing change), and mirrored in docs/changelog.md
• I have updated documentation if needed (CLAUDE.md, CONTRIBUTING.md)