Add regression coverage for broken README image paths

[Maneek21]

Current state

Refreshed September 24, 2026 against master at 1427f66.

The public-docs checker already includes README.md, validates local Markdown links (including current image syntax), and reports missing targets. CI runs it through pnpm docs:check. The remaining gap is a regression fixture, not a new checker.

Evidence: checker, CI; delivered through #330.

Remaining work and acceptance evidence

• Validate current local README image paths and clearly report a missing target.
• Run the checker in documentation CI.
• A disposable fixture with an existing local image passes.
• A missing or renamed local image fails with a nonzero exit and identifies the README and missing path.
• The regression coverage runs in the documentation CI path.
• Record the focused test command and result in the fixing PR.

Scope

File existence only. Detecting whether screenshots match the current UI is separate work. Do not overwrite or remove repository screenshots to construct a failing fixture.

Ownership and priority

Maintainer: @Maneek21. Next preview hardening; small documentation guardrail.