PRD: add FR-78 per-requirement test-coverage map (reported, per-build, justified-SKIP≠gap)

[cahenesy]

Adds FR-78 to the Verification section: for each landing TDD, throughline produces a
verification-status map of that TDD's in-scope requirements — each classified pinned /
proposed / justified-no-surface / unverified-gap — that cannot read falsely-green. It
is REPORTED for the human PR review (not an automatic flip-blocker; the FR-15 gates
remain the auto-authority), scoped per-build (no retroactive whole-system audit), and
distinguishes a justified no-observable-surface SKIP from a real gap (NFR-4 honesty).

Closes the gap FR-15(a)/FR-23/FR-71 leave: FR-15(a) gates test-PRECEDES-impl
(per-commit discipline, not per-requirement coverage); FR-23 states INTENDED
observation points (pre-build design intent); FR-71 reports actual diff/scope, not
per-requirement verification status.

Non-goals added: a pre-build de-risking spike (considered, explicitly excluded — the
build is the cheapest test for engineering design); the coverage map as a hard
auto-gate or whole-system audit. Strictly WHAT (no format/location/which-gate/which-
mechanism — those are /tdd-author's job).

Cascade: none — FR-78 is purely additive (a new requirement). It needs a covering TDD
via /tdd-author, but no existing TDD or accepted ADR becomes stale.

Open assumptions & waivers:

• redundant with FR-15(a)/FR-23/FR-71 — resolved: FR-78 is the falsifiable
  per-requirement POST-BUILD verification-status map those three leave uncovered
  (precedence-discipline / intended-plan / actual-diff-report respectively).