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

[Djain912]

Summary

Implements issue #25 by supporting ordinal queries to select an element by position among matching candidates.

What changed

• Added ordinal query parsing for first, second, third, numeric ordinals (1st, 2nd, 3rd, ...), and last.
• Added guard logic to avoid false positives for semantic phrases (for example, first name).
• Added deterministic tie-breaking in combined matcher.
• Added ordinal selection in document order after matching.
• Added parser and integration tests for normal, edge, and out-of-range ordinal cases.
• Added docs/examples in README and CLI reference.

Example queries now supported

• second button
• third menu item
• last input field

Validation

• go test ./internal/engine -run "TestParseOrdinalConstraint_BasicPatterns|TestCombinedMatcher_OrdinalQuery_SecondButton|TestCombinedMatcher_OrdinalQuery_LastInputField|TestCombinedMatcher_OrdinalQuery_OutOfRangeReturnsNoMatch|TestCombinedMatcher_OrdinalGuard_DoesNotTreatFirstNameAsOrdinal" -count=1
• go test -count=1 ./...
• go vet ./...
• go build ./...
• go build -o semantic.exe ./cmd/semantic
• go run github.com/golangci/golangci-lint/v2/cmd/golangci-lint@v2.9.0 run
• Manual CLI checks with duplicated controls:
  • second button -> second result by order
  • last input field -> last result by order
  • fifth button (out-of-range) -> no result

Environment note

• Local Windows race run (go test -race ./...) is blocked by local 64-bit cgo toolchain availability (cc1.exe ... 64-bit mode not compiled in).
• CI runs on Ubuntu and will execute race tests there.

Closes #25