Triage advisory external-link check findings

[josecelano]

Goal

Classify persistent failures from the advisory External Link Check, repair a small evidence-selected set of stale references, and add narrowly scoped online-only exclusions only for URLs that automated online validation cannot meaningfully check.

Baseline

External Link Check run 34347690674 ran on develop revision 7abc30b2b9fb85b235e7b2ef2a40f5d5f7ed1555. Its report artifact contained 1,677 checks, 1,216 successes, 24 redirects, 461 errors, and no timeouts: 440 generic errors, 18 404s, and 3 403s.

The report includes dynamic GitHub review-comment fragments, intentionally unavailable localhost examples, stale repository/docs.rs references, and third-party access or fragment limitations. These categories need separate dispositions; no broad host-level suppression is acceptable.

Scope

• Record issue-local baseline and a disposition for every distinct URL or recurring failure pattern.
• Repair bounded, verified stale references.
• Add only documented, online-only exclusions for durable false-positive categories.
• Rerun the hosted workflow after each remediation slice and retain evidence.

Constraints

• Keep the workflow scheduled/manual and visibly failing for remaining errors.
• Do not use continue-on-error, broadly exclude external hosts/fragments, replace Lychee, or change the shared linter integration.
• Do not close Enforce local Markdown links and schedule advisory external link checks #2162 or move its specification; batch issue cleanup remains separate.

Verification

Run linter all, validate changed configuration/workflows, and manually dispatch the workflow. Each exclusion must be proven narrow: its intended category disappears while unrelated external failures and the report artifact remain visible.

Parent EPIC: #2003
Enabling issue: #2162

[josecelano]

Reopened after PR #2197 merged because its reference closed this issue before the planned work was complete. The C1/C9 online exclusion is merged, but its required upstream GitHub Actions manual-dispatch verification—including report-artifact retention and confirmation that unrelated failures remain visible—is still pending. The issue will remain open until that evidence and the remaining planned triage work are completed.

[josecelano]

Correction to the previous comment: PR #2197 used Closes #2185, which auto-closed this issue prematurely. It is reopened because the required upstream GitHub Actions manual-dispatch verification remains pending, including report-artifact retention and confirmation that unrelated failures remain visible.