Define the renderer-font record schema and deterministic writer

[Happypig375]

Parent: #320

Define the deterministic renderer-font record envelope before extracting full face data.

Scope:

• add a versioned little-endian logical record header and section directory with explicit offsets/counts
• retain Prototype deterministic bundled-font artifact formats #319 source hashes and reject source drift
• provide bounds-checked generator-side writer and standalone reader that needs no SFNT/OpenFontReader parser
• define deterministic binary and generated-C# representations from one logical model
• keep production rendering unchanged

Acceptance criteria:

• synthetic and all-three-face identity/header fixtures round-trip byte-for-byte
• invalid schema, endian, offset, count, overlap, truncation, source hash, and trailing bytes fail clearly
• section and face ordering is deterministic
• binary and generated-C# readers produce equal logical headers/directories
• schema/compatibility and AOT/trimming rules are documented