refactor(arch): extract TemplateEngine with single-pass registry (#211)

[pajoma]

Summary

• Extracts resolveDate and toMomentFormat into src/journal/template-engine.ts with a central TEMPLATE_VARIABLE_MAP registry — adding a new template variable now requires editing exactly one place
• Deletes both parallel switch/case blocks from src/util/dates.ts and the third duplicate from src/test/direct/path-parse-with-date.ts
• Fixes the ${week} gap in toMomentFormat (was silently dropped; now maps to 'w')
• Thread-safe locale handling: moment(date).locale(loc) replaces global moment.locale() mutation
• Single-pass regex.replace(callback) replaces multiple forEach + string.replace calls

Closes

Closes #211

Spec / Plan

• docs/specs/2026-05-18-211-template-engine-dry.md
• docs/plans/2026-05-18-211-template-engine-dry.md

Test plan

• src/test/suite/template-engine.test.ts — 21 new assertions covering all 7 named variables, custom ${d:fmt}, locale isolation, toMomentFormat mappings including ${week}, edge cases
• Existing replace-variables-in-string.ts and path-parse-with-date.ts direct tests updated to import from new engine — behavior unchanged
• npx tsc --noEmit passes clean
• npm run compile (esbuild) passes clean
• All 21 behavior assertions verified via direct node execution

🤖 Generated with Claude Code

[pajoma]

Architecture Review: Final Approval (PR #217)

I have performed a critical architectural review of this PR. The implementation is exceptionally clean and rigorously adheres to the mandates set during the planning phase.

Key Achievements:

• Definitive DRY: Successfully consolidated three parallel, drifting implementations into a single, high-performance registry. This is the definitive fix for the Knowledge Duplication risk identified in our audit.
• Structural Consistency: The registry pattern ensures that momentFormat and actual resolve logic can never drift apart again.
• Bug Resolution: Correctly identified and fixed the missing ${week} mapping in the path-preview logic, closing a long-standing functional gap.
• Safety & Performance: The transition to isolated locales (moment().locale()) and single-pass regex replacement provides both thread-safety and (N)$ efficiency.
• Test Integrity: The comprehensive new unit test suite, particularly the locale isolation tests, provides the high-confidence verification needed for core domain services.

This refactor significantly raises the architectural standard of the project. LGTM.

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Note: Gemini was involved in creating this comment (but not without consulting me).