Adopt upstream-first gamut policy, permanent support matrix, and MSRV policy

[justin13888]

• Add "Upstream-First Policy (gamut)" section to AGENTS.md (CLAUDE.md is a symlink); update PRINCIPLES.md: name gamut as the default home for image primitives/codecs/containers/metadata + new safety boundary for rawshift-hwdec.
• Add README MSRV policy: MSRV tracks the minimum required by upstream dependencies (currently 1.92 via gamut); never raised independently.
• Create docs/SUPPORT.md: permanent compilation-target + hardware-API matrix with written justifications for every exclusion (Windows MF, NVDEC, D3D12/Vulkan Video).
• Create docs/V1_DESIGN.md: full v1 shape (features, configs, API, compile boundaries).
• Labels epic/blocked-upstream exist.

[justin13888]

Delivered by 9a0c3b3 on master. Verified complete against all five acceptance items: the AGENTS.md upstream-first section (CLAUDE.md symlink intact), PRINCIPLES.md naming gamut as the home for primitives/codecs/containers/metadata plus the rawshift-hwdec safety boundary, the README MSRV policy, docs/SUPPORT.md with written justifications for all three named exclusions (Windows MF, NVDEC, D3D12/Vulkan Video), docs/V1_DESIGN.md, and both the epic and blocked-upstream labels existing. Ready to close.