feat(theme-selector): add package-face state and schema (tier-3 phase 1)

I laid the tier-3 foundation: an APPS registry (org starter for now) and a PKGMAP holding {fg,bg,bold,italic,inherit,source} per face. Pure helpers seed PKGMAP from APPS defaults, build the export per the state policy, and merge an import back in. Export gains a packages key when any package face is present, and import reads it while old JSON with no packages key still loads cleanly onto the seeded defaults. No UI yet — that's phase 3.

A #selftest harness, guarded by the URL hash so it never shows in normal use, runs seed to export to import and checks the round-trip, old-JSON merge, and inherit/source survival. Headless Chrome reports PASS, which is how I verified the schema end-to-end against the real emitted code rather than a copy.

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