ci: run tests with a coverage floor on every PR (refs #585)

[Zeroupper]

Problem

test/ is thin and nothing enforces growth; CI work is already tracked in #585 — this adds the test gate only (note: the workflow files were not audited in depth).

Proposed fix

PR workflow runs flutter analyze + flutter test --coverage with a ratchet-only coverage floor (starts at current %, never decreases).

Acceptance criteria

• PR fails on test failure or coverage drop
• Badge in README

Effort: S · Priority: P2 · Change risk: None