feat: harden matchers with input sanitization and context cancellation

[luigiagent]

Summary

Hardens matcher behavior around invalid or extreme inputs, and improves cancellation handling.

What changed

• input sanitization and bounds handling across matcher flows
• context cancellation hardening
• added hardening-focused tests
• includes the currently stacked benchmark/visual/ranking work already present beneath this branch

Validation

• go test ./... -count=1

Notes

• This branch is stacked on top of the recent benchmark/visual work, so the diff is broader than the top commit alone.
• Supersedes hardening: normalize fusion weights and bound combined scores #32 with the same hardening direction carried forward on top of newer branch history.