feat: support ordinal queries (first/second/last)

[luigiagent]

Summary

This PR supersedes #30 and delivers the ordinal-query work for #25 in a cleaner follow-up branch.

It adds support for selecting elements by ordinal position among matching candidates, including:

• first, second, third
• numeric ordinals like 3rd
• last
• composition with context exclusion, for example second submit button not in login

What changed

• added ordinal query parsing and selection flow
• preserved document order for ordinal queries, including snapshots without explicit positional hints
• added guard behavior so literal phrases like first name are not misparsed as ordinal intent
• composed ordinal selection with context exclusion logic
• added unit coverage for ordinal parsing and matching behavior
• added ordinal corpus benchmark data and e2e coverage
• cleaned up the local dev command from ./dev benchmark to ./dev bench while keeping compatibility

Validation

• go test ./... -count=1
• ordinal e2e case passes with the repo-built binary
• ./dev bench

Notes

• Supersedes feat: support ordinal queries (first/second/last) #30
• Closes Support ordinal queries (first, second, last) #25