[alpha.7] Raise test coverage threshold to 75% and add branch/function coverage gate

[SisyphusZheng]

The project currently sits at 71.2% line coverage with a 70% threshold, leaving almost no margin before CI fails.

Acceptance:

• Raise the line-coverage threshold from 70% to 75%.
• Add branch and function coverage reporting to deno task test:coverage.
• Add a CI gate that fails when branch/function coverage regresses.
• Keep deno task test:coverage:check green.

Context: tools/check-coverage.ts, deno.json test:coverage tasks.