Support mishearing-to-correction mappings in custom vocabulary (closes #125)

[swimmer-303]

Split out from #254.

Vocabulary entries can map heard forms to the intended term with an arrow — cloud code -> Claude Code — and multiple heard forms can share one correction: cloud code | clod code -> Claude Code. Pairs are passed to the post-processing model as an explicit corrections block (with a guard against over-applying); plain entries behave exactly as before. Parser verified against plain entries, single/multi-variant mappings, mixed comma-separated lines, =>, and malformed input. Settings hint text and README document the syntax.

Closes #125

🤖 Generated with Claude Code

Summary by CodeRabbit

• New Features

  • Custom vocabulary now supports correcting specific mishearings with arrow mappings, such as cloud code -> Claude Code.
  • Multiple heard variants can share one correction using |.
  • Corrections are applied when context indicates the intended term.

• Documentation

  • Updated the settings guidance, README, and changelog with the new vocabulary syntax.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro Plus

Run ID: 8d5974a0-50e6-4706-9100-53ed4cd7e168

📥 Commits

Reviewing files that changed from the base of the PR and between 7427ca9 and dc42e65.

📒 Files selected for processing (4)

• CHANGELOG.md
• README.md
• Sources/PostProcessingService.swift
• Sources/SettingsView.swift

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📝 Walkthrough

Walkthrough

Custom vocabulary now supports explicit heard-to-corrected mappings, multiple heard variants, and malformed-entry fallback handling. The post-processing prompt, settings guidance, README, and changelog document and use the new syntax.

Changes

Custom vocabulary mappings

Layer / File(s)	Summary
Mapping parsing and prompt integration Sources/PostProcessingService.swift	Parses heard -> correct and heard => correct mappings, supports `
Input guidance and documentation Sources/SettingsView.swift, README.md, CHANGELOG.md	Documents arrow-based corrections and multiple heard variants while retaining plain vocabulary entries.

Estimated code review effort: 3 (Moderate) | ~20 minutes

Sequence Diagram(s)

sequenceDiagram
  participant SettingsView
  participant PostProcessingService
  participant LanguageModel
  SettingsView->>PostProcessingService: custom vocabulary entries
  PostProcessingService->>PostProcessingService: parse heard-to-corrected mappings
  PostProcessingService->>LanguageModel: prioritized terms and known mishearings prompt

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🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 33.33% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly summarizes the main change: adding mishearing-to-correction mappings in custom vocabulary.
Linked Issues check	✅ Passed	The PR implements explicit heard-to-correct mappings, supports multiple heard variants, and keeps plain entries backward compatible as requested in #125.
Out of Scope Changes check	✅ Passed	The docs and prompt wiring changes stay aligned with the mapping feature and documentation requirements; no unrelated scope is evident.

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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