fix(#4699): skip already-complete phases in the next_phase cascade (#4820)

* test(#4699): add failing-first coverage for skipping complete phases in next_phase

* fix(#4699): skip already-complete phases in the next_phase cascade

Both next-phase scans selected the numerically lowest phase above N
without consulting completion state, so completing a reopened phase
persisted an already-[x] phase as STATE.md current_phase while
roadmap.analyze correctly named the outstanding one (issue repro:
completing 2 with phases 1 and 3 already [x] returned next_phase 03).

The cascade collects the complete phase numbers from the roadmap
checkboxes (milestone-scoped, comparePhaseNum-deduped) and skips them in
both the disk scan and the roadmap scan; a [x] checkbox row and its
heading sibling both name a phase that is never next. Heading-only and
checkbox-less roadmaps behave exactly as before.

* test(#4699): align the negative-control expectation with the disk spelling

* test(#4699): pin the STATE.md persistence and the all-later-complete tail corner

Review findings: the regression never asserted STATE.md current_phase
(the issue's actual harm), and the all-later-phases-[x] corner
(is_last_phase true, next_phase null) was unpinned. A changeset fragment
is included.

* docs(#4699): backfill changeset PR number

---------

Co-authored-by: sim <sim@local>

.changeset/daring-finches-zip.md

@@ -0,0 +1,5 @@
+---
+type: Fixed
+pr: 4820
+---
+**phase complete no longer names an already-complete phase as next** — completing a reopened phase out of order (later phases already [x]) picked the numerically-next phase even when its checkbox was already ticked, persisting it to STATE.md as current_phase. next_phase now skips phases whose roadmap checkbox is already [x], agreeing with roadmap.analyze and init.progress. (#4699)