Redesign Chroma around configurable comptime themes

[adia-dev]

What changed

• redesigned the formatter around explicit namespaced directives such as {#red,bold}
• added typed ZON themes, semantic styles, configurable syntax, ANSI/plain variants, and terminal capability detection
• replaced repeated comptime string concatenation with an exact-size two-pass renderer
• expanded color, effect, reset, compile-error, UTF-8, and stress coverage
• added a runnable configurable example, real terminal screenshot, benchmark, migration guide, and comptime/runtime table
• updated package metadata and CI for Zig 0.16.0 across Linux, macOS, and Windows

Why

The old parser could confuse Chroma directives with Zig format fields, produced redundant resets, and was difficult to extend safely. This v0.2 redesign keeps the library comptime-first while making configuration, diagnostics, and cross-platform behavior much clearer.

This is a breaking syntax update: {red} becomes {#red}, with a migration table in the README.

Validation

• zig fmt --check .
• zig build test
• zig build test -Doptimize=ReleaseFast
• zig build
• zig build benchmark
• cross-builds for x86_64/aarch64 Linux, x86_64 Windows, and aarch64 macOS
• vhs validate docs/chroma.tape