policy-catalogue: add Mesa entry

[sjh9714]

Summary

Adds policy-catalogue/mesa.md, closing #13. Source is docs/submittingpatches.rst on freedesktop GitLab, pinned to 7a6f569.

Mesa has the most machine-readable disclosure convention in the catalogue, and I think it should drive the shape of the disclosure field. It specifies a two-tier trailer vocabulary — Assisted-by: TOOL (OPTIONAL: MODEL) when AI helped decide or generated parts, Generated-by: TOOL (OPTIONAL: MODEL) when almost all the code was generated — and explicitly reserves Co-authored-by: for humans. The v0.1 granularity: tool-and-role | role-only | binary doesn't reach this: it's tool and model and a degree tier, in a fixed grammar, adoptable with no new infrastructure.

Two other things: Mesa is the third project to legislate on commit trailers and the three do not agree (Fedora mandates Assisted-by:, Kubernetes forbids it, Mesa mandates it while banning Co-authored-by:) — the fragmentation is already encoded in commit metadata CI could enforce today. And its disclosure exemptions are drawn at the limit of distinguishability ("would be the same regardless of what tooling was used"), which is the same boundary Bevy's [^1] footnote reaches from the prohibition side. Two independent arrivals at the same line.

AI disclosure (required — we dogfood our own standard)

• AI-assisted — tool and role: drafted by Claude (Claude Code) working under my direction. Every verbatim excerpt was verified programmatically against the pinned source before submission (26 string checks across this batch of five entries, all passing).

Attestations (required)

• understands-change — I can explain every part of this entry and the sources behind it.
• reviewed-output — reviewed before submission. Being precise about the mechanism rather than just ticking: I reviewed a structured walkthrough of the entry plus the verification output confirming each quoted string exists at the pinned source, rather than reading the rendered diff line-by-line.
• ran-tests — N/A (prose only).

Type of contribution

• Policy-catalogue entry (single file in policy-catalogue/)

I've left policy-catalogue/README.md untouched across this batch — #21 already edits that table and several PRs touching the same rows would conflict. Happy to send one follow-up PR updating the count and all rows once merge order is settled.

Maintainership disclosure: I maintain MergeWarden, a checker in the layer-3 space discussed in #20/#22. This entry documents the project's policy only and makes no claim about any tool.