fix: nightly hardening - finite signal number validation

[mouse-value-add]

Problem

Signal extraction accepted non-finite numeric values (for example Infinity) when parsing profile fields. This can propagate invalid numeric state into personalization logic and downstream ranking behavior.

Approach

• Hardened toNumber in src/core/signals.ts to only accept finite numbers for both numeric and string inputs.
• Added a regression test in test/core/signals.test.ts to verify trust_score: Infinity is dropped instead of emitted as a signal.

Verification

• Ran full test suite: npm test
• Result: 17 test files passed, 145 tests passed.

Risks

• Low risk. Change only filters invalid non-finite numbers.
• Potential behavior change for any caller that previously relied on non-finite values (undesirable behavior).

Rollback Plan

• Revert commit 1ed0868 to restore prior parsing behavior.