visitAllLinks: production-faithful navigation for relative and hash links; linear crawl

[NullVoxPopuli]

Three fixes, found crawling AuditBoard's 265-page documentation app (follow-up to #131):

1. Relative hrefs navigate where production would — and warn

The click navigates by element.href, which the browser resolves against the test page's URL (e.g. /tests) — not the app's current route the way a production visit would (there, the address bar is the current route). Verified in the harness: clicking ../data-schema/index.md from /schema/getting-started/index.md sent the router to /data-schema/index.md instead of /schema/data-schema/index.md.

The crawler already resolves each link's target against currentURL() at the moment it encounters it — that's what the expected value is built from. Now it also points the anchor at that resolved target before clicking, so the click exercises the real properLinks-and-router path with the URL a production visitor would produce. (Absolute hrefs are left untouched.)

Because a relative href is an authoring hazard (it only behaves when the address bar is the app's current route), the rewrite also emits a console.warn naming the page, the offending href, and the exact root-absolute path to author instead — an action item for fixing the source document.

2. Hash-insensitive navigation assertion

currentURL() never includes a #hash, so page#section links always read as failed navigations (/hash-target doesn't start with /hash-target#down). The assertion now compares without the hash.

3. Linear crawl

visited was keyed on (current page, target) pairs, so every target was re-visited from every page that links to it. Shared nav links appear on every page, making the crawl quadratic in app size — ~13k visits for the 265-page app, which times out any sane assert.timeout. It's now keyed on the target alone: one visit per reachable URL, which is what "are all links visitable" means.

Tests: the test-app gains a nested route whose template has a relative <a href="other"> (the crawl hard-errors without fix 1; the test also asserts the warning fires and names the resolved target), a page#hash link to an otherwise-unlinked route (fails without fix 2), and an assertion that no target is visited twice. 8/8 passing with the fixes; the suite errors without them.

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