docs: document offline proof lane platform and font prerequisites

[doublewhy]

Documentation gap

The contributor guide documents the canonical verification command but not that the participant-opacity proof lane requires Linux x86_64, Bubblewrap, fontconfig, and an installed font. On a clean Linux host, a missing runtime prerequisite can therefore be mistaken for a rejected theorem.

Expected documentation

• State the supported proof-runner platform.
• List Bubblewrap, fontconfig, and a minimal font package.
• Explain that restrictive namespace policy produces a sandbox-unavailable result and must not be bypassed by weakening offline isolation.
• Direct non-Linux contributors to canonical CI.

Scope

This issue covers contributor documentation only. Proof-tool preflight and Ubuntu sandbox portability remain tracked in #1109.

Related implementation

Proposed in #1088.

[doublewhy]

Completed by #1131, squash-merged to dev as 990e8fbc246769fd95f4100e225a2eb69e2812f3. The merged documentation records the offline proof platform, font, sandbox, and replay prerequisites; exact-head docs, verification, and Sonar passed. Closing manually because merges to non-default dev do not trigger GitHub closing keywords.