Prepare v2.7.3 release

[pttydou]

What changed

• finalize the macOS settings light-card contrast repair as version 2.7.3
• fix transparent forest-day and cavern Composer backgrounds on Windows and macOS
• keep a themed fallback when future biome variables are missing
• keep Windows and macOS release identities synchronized
• retain the verified Windows Composer outline removal and environment-music continuity fixes

Root cause

The final biome card rule used background: var(--biome-card) for forest-day and cavern, but those two special variants did not define --biome-card. CSS invalidated the winning background declaration, leaving the Composer transparent and exposing Codex native dark gray.

Validation

• node release/validate-release.mjs --tag v2.7.3
• Windows full source test suite
• macOS Node regression suite and injector payload check
• Windows live DOM: forest-day and cavern resolve to themed gradients, outline is none, one active Composer marker
• hosted Windows, macOS, Ubuntu, and macOS native build checks required before merge