[terehov]

Supersedes #380 by @clayzx (cherry-picked with their authorship; thank you!) with a follow-up commit addressing the review.

Feature

pretty.placeholders registers custom {{token}}s for pretty.template. Each callback gets (meta, pretty) and returns a string or number; style the token through pretty.styles like a built-in, so a composite value such as HH:mm:ss.SSS is colored as one unit. Only referenced tokens run, a throw renders empty (never drops the log), built-in names are reserved, sub-loggers inherit the parent's map. Meta template only — error templates and JSON output are unchanged.

new Logger({
  pretty: {
    template: "{{shortTime}} {{logLevelName}} ",
    placeholders: { shortTime: (meta) => meta.date.toISOString().slice(11, 23) },
    styles: { shortTime: "blackBright" },
  },
});

Follow-up on top of #380

• Callbacks may return numbers as well as strings.
• IPrettyLogStyles accepts any key so custom tokens can be styled; a misspelled built-in key is therefore reported at runtime instead (UNKNOWN_PRETTY_STYLE with a did-you-mean; dev warning, throws under strictConfig).
• KNOWN_PRETTY_PLACEHOLDERS moved to src/internal/prettyPlaceholders.ts so internal/ no longer imports core/settings.
• Docs: README ### Custom placeholders subsection + settings table, RECIPES 16, llms.txt pretty bullet, CHANGELOG 5.3.0 entry.
• Browser bundle budget 22,400 → 23,000 bytes (measured 22.24KB gzip; the feature plus its validation).

Verification

Vitest 1693 tests at 100% coverage, Biome, check-doc-sync, audit-sideeffects, build, Deno adapter, Bun, and all 321 Playwright tests on Chromium/Firefox/WebKit.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (a3699f9) to head (924f02e).
⚠️ Report is 9 commits behind head on development.

Additional details and impacted files

@@              Coverage Diff              @@
##           development      #386   +/-   ##
=============================================
  Coverage       100.00%   100.00%           
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  Files               53        54    +1     
  Lines             4632      4660   +28     
  Branches          1417      1433   +16     
=============================================
+ Hits              4632      4660   +28     

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