Add DateTime::format_with token DSL

[brickfrog]

Closes bead tempo-dwz.4.

Adds DateTime::format_with(pattern) for brace-token custom UTC DateTime formatting with literal passthrough, doubled-brace escapes, fixed-width millisecond/nanosecond tokens, and TempoError rejection for unknown tokens.

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: 9e03938
  • output tail:

Finished. moon: no work to do

• moon test --target wasm-gc
  • exit: 0
  • head: 9e03938
  • output tail:

Total tests: 287, passed: 287, failed: 0.

• moon test --target js
  • exit: 0
  • head: 9e03938
  • output tail:

Total tests: 287, passed: 287, failed: 0.

• moon test --target native
  • exit: 0
  • head: 9e03938
  • output tail:

Total tests: 287, passed: 287, 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 formatting API for DateTime that supports a small brace-token DSL, enabling custom UTC timestamp rendering beyond the existing RFC3339-style DateTime::format().

Changes:

• Introduces DateTime::format_with(pattern) with brace-token parsing, brace escaping, and TempoError for invalid tokens.
• Adds test coverage for token substitution, literal passthrough, escaping, and unknown-token rejection.
• Updates the generated public interface (pkg.generated.mbti) to expose the new API.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
src/tempo.mbt	Implements DateTime::format_with and token dispatch helper.
src/tempo_test.mbt	Adds blackbox tests validating formatting output and error behavior.
src/pkg.generated.mbti	Exposes DateTime::format_with in the generated public API surface.

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[brickfrog]

Choir: comment-only review received — routed to main.ergo-task-add-datetime-formatwith-1780418501483-596086 (leaf pane).

[brickfrog]

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

[brickfrog]

Choir: fixes pushed by main.ergo-task-add-datetime-formatwith-1780418501483-596086 — resolving threads and checking gates (not merged yet).

[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).

[brickfrog]

Choir: fixes pushed by main.ergo-task-add-datetime-formatwith-1780418501483-596086 — resolving threads and checking gates (not merged yet).

[brickfrog]

Choir: fixes pushed by main.ergo-task-add-datetime-formatwith-1780418501483-596086 — resolving threads and checking gates (not merged yet).

[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).