fix: close code-review 330-r1 — budgets from the slowest machine, the bench in Validate, AC1 through the execution thread (#330)

H2: the benchmark budgets were calibrated on the author's sandbox with a
1.14x margin — the review runner measured tsFullCandidateMs at 169-171 ms
against a 100 ms ceiling. Budgets now keep the spec's 2-3x allowance over
the SLOWEST observed machine, and the benchmark runs as a step of the
Validate perf job on every push (it needs no browser and no bundle), not
only inside the weekly mutation gate.

M1: the promised AC1 backend test exists now and does what AC1 means: it
patches validate_junction_limits with a thread-recording wrapper inside the
real HA harness — on the event loop that would be MainThread — and proves
the verdicts survived the move (a clean write is accepted, a write adding a
spike is refused with junction_limit_angle). Spec revision 4 rewrites AC1
around this invariant instead of a fragile millisecond assertion.

M2: §4.6 equivalence is now behavioural on both sides (three boundary
fixtures each: as-is counts equal through-migration counts, TS and python),
and the parity suite gained the §7 boundary fixtures (exact 15°, exact
20 cm, the thickness-step filler run, exact 5 cm).

H1 was already closed by 7513f93d (the review ran on the previous HEAD):
check-docs is green on this tree — the screenshots and their manifest come
from one capture run.

Issue: #330
User-Visible: no

.github/workflows/validate.yml

@@ -543,6 +543,10 @@ jobs:
       - name: Enforce absolute smoke ceilings
         run: |
           npm run benchmark:compare -- --absolute-only --budgets=demo/performance/budgets-glow-smoke.json --candidate=artifacts/performance-smoke/candidate.json --output=artifacts/performance-smoke/comparison.json
+      # #330 AC7: перф-контракт ограничений стыков — быстрый (без браузера),
+      # ловит возврат квадратичных путей в оба зеркала прямо на пуше.
+      - name: Бюджеты ограничений стыков (#330)
+        run: npm run benchmark:junction-limits
       - name: Upload performance smoke report
         if: always()
         uses: actions/upload-artifact@v7