Roadmap lint cannot check a Blocked-by decision anchor in a non-Markdown file #3567

Problem

A project that keeps decisions as entries in YAML files can declare it in .specfuse/layout.yml:

decisions:
  style: entries
  location: decisions
  link: "{location}/{domain}.yaml#{id}"

The roadmap lint then passes a typo in the fragment of a Blocked-by link:

**Blocked by.** [pricing-is-per-sat](../decisions/billing.yaml#pricing-is-per-sat) — awaiting signature.

_check_blocked_by in specfuse/loop/lint_roadmap.py checks that the file exists and sits under decisions.location, but checks the fragment only for Markdown targets (target.suffix == ".md"). A renamed or withdrawn decision passes the same way, so the one check that ties a blocker to a specific decision is off for the projects style: entries was added for.

Proposed

• Add an opt-in decisions.entryKey naming the key that holds an entry's id. Without it, behavior stays as today.
• Report a missing fragment as an ERROR under status: blocked, worded like the Markdown finding: … points at '<file>', which has no anchor '<id>'.
• A decision still under review links to the file with no fragment, or to its PR; both already pass. The link gains its #id when the entry lands.
• If the target's shape cannot be read for entryKey, report a clear finding, never a silent pass.

Notes for the fix

• Do not parse the whole file with _miniyaml. It rejects block scalars and flow mappings by design (literal/folded block scalars unsupported), and decision files can hold both. The loop has no runtime dependencies, so scan lines for entryKey: <value>, and use stdlib json for JSON.
• Rows not in blocked never reach these link checks; a leftover Blocked-by block already gets a WARN, which covers a forgotten unblock.
• specfuse/loop/layout.py also changes in FEAT-LOOP-2026-0249: changelog fragments so concurrent PRs stop conflicting #3566, so start after that merges. Size: one bug fix.