Add allocation-aware scanner contracts and shared text hints

[alyldas]

Context

Introduce shared scanner contracts in the core package so downstream packages can align on one result shape and reuse allocation-aware helper paths.

Scope

• Add shared ScanInput, ScanHints, TextRange, and ScanResult contracts.
• Add shared helper flows for check, scan, and censor with avoidable allocations minimized.
• Preserve the existing public API through backward-compatible wrappers or aliases where needed.
• Keep package implementation contained in this package; do not move downstream scanner logic here.

Acceptance Criteria

• Existing public API remains backward compatible.
• New contracts are exported and documented through the package's existing public surface conventions.
• Shared helpers can support early-exit check, full scan, and censor use cases.
• Tests cover compatibility and the new shared contract/helper behavior.
• Benchmark evidence demonstrates the allocation-aware path does not regress the relevant baseline.

Validation

• npm ci
• npm run check
• Relevant unit/regression tests
• Relevant benchmarks
• npm pack --dry-run

Compatibility Notes

This should be a backward-compatible additive change unless implementation discovers an existing contract conflict that needs an explicit note before PR review.