feat: add Duration humanize

[brickfrog]

Closes bead tempo-z7j.5.

Adds Duration::humanize for deterministic English elapsed formatting over day/hour/minute/second components, including negative durations, sub-second zero output, and the Int64::min_value guard.

Verification:

• moon fmt --check && moon test --target wasm-gc && moon test --target js && moon test --target native

Verification

Generated by Choir from commands executed in the leaf workspace.

• moon fmt --check
  • exit: 0
  • head: bf56b67
  • output tail:

Finished. moon: no work to do

• moon test --target wasm-gc
  • exit: 0
  • head: bf56b67
  • output tail:

Total tests: 285, passed: 285, failed: 0.

• moon test --target js
  • exit: 0
  • head: bf56b67
  • output tail:

Total tests: 285, passed: 285, failed: 0.

• moon test --target native
  • exit: 0
  • head: bf56b67
  • output tail:

Total tests: 285, passed: 285, failed: 0.

[brickfrog]

Choir: GitHub Actions rollup reports all checks successful on the latest head — re-check review/merge policy on GitHub (not an approval or merge signal by itself).

[Copilot]

Pull request overview

Adds a new public Duration::humanize formatter to the Tempo core module, providing deterministic English “elapsed time” strings composed from day/hour/minute/second components (dropping any sub-second remainder) while handling negative durations and the Int64::min_value edge.

Changes:

• Introduce Duration::humanize() and an internal helper to emit pluralized English components.
• Add unit tests covering multi-component formatting, pluralization, negative durations, sub-second truncation to zero, and Int64::min_value.
• Export the new API in the generated package interface (pkg.generated.mbti).

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
src/tempo.mbt	Adds Duration::humanize and a helper to build English elapsed-time strings from whole seconds.
src/tempo_test.mbt	Adds test coverage for Duration::humanize, including edge cases (negative and Int64::min_value).
src/pkg.generated.mbti	Exposes Duration::humanize(Self) -> String in the public API surface.

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[brickfrog]

Choir: GitHub Copilot issue-thread comment observed on PR — parent notified with updated gate snapshot (not merge approval).