route: remove two host advantages instead of disclosing them

[MikeyPetrillo]

Follow-up to #660, which published three rules favouring our own catalog on the theory that a checkable small claim beats an unverifiable large one. Right instinct, applied one step too late: two of the three did not need to exist.

Removed

The per-seller diversity cap now applies to our catalog on the same terms as every external seller's. The exemption was justified on the reasoning that capping the host pushes buyers toward less-vetted externals. Measured over 30 representative queries at top=12, our catalog took 8.3% of slots and the exemption bound on exactly one query. It was buying almost nothing and costing the thing the endpoint exists for.

Skipped only for include=local, where there is one seller by definition and a per-seller cap would truncate the answer to a third. Verified: include=local&top=12 still returns 12.

The listing-injection filter now runs against our rows too. It was external-only because the filter defends against seller-controlled text and we trust our own, but "we are exempt from our own safety check" is a rule that favours the host, and a catalog entry of ours that tripped it would be a bug worth seeing rather than an exception worth granting. Verified live: all 526 tools still findable by their own slug, none filtered.

Kept, and still disclosed

Our own health is asserted as 1 rather than measured, because the crawler never probes itself and there is no honest way to synthesise a crawl history for ourselves on the router's hot path. Every result carries why.healthSource so an asserted 1 is never mistaken for a measured one.

The neutrality block went from three entries to one, by fixing rather than explaining.

Verification

• 80 assertions in scripts/test-discovery-note.js, in CI
• The tests now guard the removals as well as the disclosure. Three mutations, all killed: restoring the cap exemption, making the filter external-only again, and disarming the filter entirely
• The filter guard pins both directions — no ordinary tool description of ours trips it, and a real injection attempt still does — so making it symmetric cannot have quietly disarmed it
• Sweeps clean: 210 strict / 296 lenient tools, 526/526 through MCP