perf: add reproducible font startup profiler

[Happypig375]

Closes #293

Summary

• add a reproducible, process-isolated font startup profiler without changing normal BenchmarkDotNet dispatch
• record first-use wall time, managed allocations/retention, GC counts, SFNT inventory, packaging sizes, exercised paths, and schema-versioned JSON evidence
• exercise Latin Modern, AMS, Cyrillic, and a runtime-loaded Comic Neue face, including a global four-face switching batch with cmap-proven representative corpora
• add a threshold-free CI smoke and documentation for repeatable local/CI comparisons

Validation

• Release benchmark project build: 0 warnings, 0 errors
• --font-profile-smoke: 10 samples, 0 report errors
• exact four-face order/corpora, cmap glyph IDs, LocalTypeface, and successful draws verified
• invalid-worker/error propagation and custom font stream lifetime verified
• independent validation and review: approved