docs: fix limitation 1 honesty after Ed25519 Default/Strict close (#523)

[IanFrelinger]

Summary

Fix stale text that incorrectly described Ed25519 signature enforcement and schema floor requirements as optional after PR #523 closed limitations 7 and 8. Two locations claimed that Default/Strict verification closed limitations 7-9, which contradicted the fact that limitation 9 (composition signer key discard) remains OPEN.

Changes

• docs/certification-evidence.md: Update limitation 1's summary sentence to accurately reflect that Ed25519 is now required on Default/Strict verification paths (limitations 7–8 CLOSED as of 2026-09-06), while limitation 9 remains open
• src/Ashlar.Certification.Contracts/CertificationVerifyOptions.cs: Fix class-level XML remarks claiming Default/Strict close "limitations 7-9" → correct to "limitations 7–8"

Both fixes preserve the accurate status: limitations 7-8 are CLOSED, limitation 9 is OPEN.

Testing

Documentation and XML remarks only. No code behavior changes.

• Verified the changes accurately reflect the CLOSED status updates in limitations 7 and 8 (detailed sections at lines 750-757 and 798-803 of certification-evidence.md)
• Confirmed limitation 9 has no CLOSED update and remains accurately described as open
• Verified CertificationVerifyOptions XML remarks now correctly reference only limitations 7-8

Testing strategy (blast radius)

Not applicable — documentation and XML remarks fixes only.

Checklist

• make test passes locally (not run - docs/remarks only changes)
• Documentation updated (this IS the documentation update)
• No TODO or NotImplementedException left unresolved (no code changes)
• Breaking changes are documented (not applicable)

Release (only when this PR ships a versioned NuGet/GHCR release)

• Not a versioned release — skip