docs: Add support-tiers.md for c2patool

[scouten-adobe]

Summary

• Add docs/support-tiers.md, ported from c2pa-rs's support tiers document and scoped to c2patool's own single-binary CI/release setup.
• Platforms where a command-line tool isn't relevant (iOS, Android, browser wasm) are omitted, per c2pa-rs's wasm32-unknown-unknown / mobile targets not applying here.
• Tier 1A configurations reflect c2patool's actual ci.yml matrix (Ubuntu/macOS/Windows × stable/MSRV, plus WASI under wasmtime) and c2patool-release.yml's published binaries. Tier 1B/2/3 are documented as currently empty, since this repo folds what c2pa-rs splits across Tier 1A/1B into a single required workflow.
• Update docs/release-process.md, which previously flagged the missing support-tiers doc as follow-up work; that note is now stale, replaced with a link.

Test plan

• Reviewed rendered Markdown links/anchors for correctness (relative links to release-process.md#validation-gating, in-page #how-the-tiers-gate-merges-and-releases anchor).
• Cross-checked Tier 1A configuration details against .github/workflows/ci.yml and .github/workflows/c2patool-release.yml.
• N/A: docs-only change, no code/build changes.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 72.16%. Comparing base (3172f88) to head (73922ca).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #313   +/-   ##
=======================================
  Coverage   72.16%   72.16%           
=======================================
  Files           4        4           
  Lines        1060     1060           
=======================================
  Hits          765      765           
  Misses        295      295           

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