Add DateTime format_fixed

[brickfrog]

Closes bead tempo-dwz.3.\n\nAdds DateTime::format_fixed for fixed-width UTC timestamps with exactly nine fractional-second digits, documents the 0..9999 lexicographic ordering guarantee, and covers whole-second, fractional padding, lexicographic ordering, and fixed-width in-range outputs.

Verification

Generated by Choir from commands executed in the leaf workspace.

• moon fmt --check
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Finished. moon: no work to do

• moon test --target wasm-gc
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Total tests: 288, passed: 288, failed: 0.

• moon test --target js
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Total tests: 288, passed: 288, failed: 0.

• moon test --target native
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Total tests: 288, passed: 288, 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 DateTime::format_fixed API to emit a UTC timestamp string with an always-present 9-digit fractional-seconds field, aimed at producing fixed-width outputs (within year 0..9999) that preserve chronological ordering under lexicographic comparison.

Changes:

• Added DateTime::format_fixed() producing YYYY-MM-DDTHH:MM:SS.nnnnnnnnnZ (always 9 fractional digits).
• Added tests covering whole-second formatting, fractional padding, lexicographic ordering (in-range years), and fixed-width (in-range years).
• Updated the generated public API surface (pkg.generated.mbti) to include the new function.

Reviewed changes

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

File	Description
src/tempo.mbt	Adds DateTime::format_fixed implementation and its docstring/guarantees.
src/tempo_test.mbt	Adds coverage for format_fixed behavior and ordering/width properties.
src/pkg.generated.mbti	Exposes the new DateTime::format_fixed signature in generated API metadata.

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[brickfrog]

Choir: comment-only review received — routed to main.ergo-task-add-datetime-formatfixed-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-formatfixed-1780418501483-596086 — resolving threads and checking gates (not merged yet).

[brickfrog]

Choir: fixes pushed by main.ergo-task-add-datetime-formatfixed-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).