Lane D R19 (v4.0-alpha close): fragmentation re-measure — A's ruling verified (ledger #3)

Post-fix re-measure of my R18 finding against A's committed connectivity pass
(6fc4b40). Same harness, same seed.

VERDICT: fragmentation fully resolved, sim untouched. Real Katoomba:
components 105 -> 1, street-metres in main 58.2% -> 100%, shops stranded 2 -> 0
(1 bridged, 1 dropped past SEAT_MAX AND counted in norm.dropped), % near-crowd on
fragments ~10% -> 0%. selfcheck 51670/51670 green, katoomba golden pinned.

Mechanism check: re-ran R18's dead-straight-through-junction geometry — R18 dropped
the junction (deg 2, disconnected); R19 protects it (deg-4 crossroads, 1 component).
DP-collinear drop fixed at the source.

Sim confirm on the fixed bridged graph: 0 NaN over 1,303,200 samples, footpath-perfect,
loiter/bench-sit fire, patronage rose 24->41 (peds reach shops across the connected
graph), 0 fragment-stranded peds. Zero sim changes, as R18 predicted.

D->F: fragmentation verdict post-fix is GREEN — tag away. FYI B/F: edges rose 799->966
(junction protection + bridges), flagged for the tris budget.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

docs/LANES/LANE_D_NOTES.md

@@ -6,6 +6,47 @@ _rotOnly/head-bone normalize/upgradeStreetPeople). Measurements on the M3 Ultra
 
 ---
 
+## ROUND 19 — v4.0-alpha CLOSE: the fragmentation re-measure (ledger #3)
+
+*PROCITY-D, 2026-07-16. A resolved my R18 finding (commit `6fc4b40`: junction-protected DP + bridge-joins
++ shopless-island culling, katoomba golden pinned). This is the post-fix re-measure — same harness, same
+seed. **Verdict: the ruling landed clean — the fragmentation is fully resolved and the sim stays untouched.***
+
+### Before → after on the real Katoomba graph
+
+| metric | R18 (my finding) | **R19 (post-fix)** |
+|---|---|---|
+| connected components | 105 | **1** |
+| street-metres in the main net | 58.2 % | **100 %** |
+| shops stranded off-main | 2 | **0** — 1 bridged to main, 1 dropped past `SEAT_MAX` **and counted** (`norm.dropped`, plan_osm.js:319) |
+| **% near-crowd on tiny fragments** | ~10 % | **0 %** (no fragments left) |
+| nodes / edges | 872 / 799 | 830 / 966 |
+
+Shops: 20 in cache → **19 seated + 1 counted-drop, 0 silently stranded** (ruling satisfied). `selfcheck`
+**51670/51670 green**, katoomba real-roads golden pinned in the output-drift guard.
+
+### The DP-collinear junction-drop mechanism is fixed at the source
+Re-ran R18's original failing geometry — a **dead-straight cross street through a collinear mid-junction**
+(the exact case B and I both hit). R18: DP flattened it, dropped the junction → **max-degree 2, disconnected.**
+R19: the junction is protected from DP → **degree-4 crossroads forms, 1 component.** Fixed.
+
+### Sim confirm on the fixed (bridged) graph — still zero changes
+Fresh stream sim, real Katoomba, drove the shop blocks: **0 NaN over 1,303,200 samples**, footpaths perfect
+(0 off-footpath, maxDist 12.9 m), 312 active peds, loiter (251) + **bench-sit (77)** fire, and **patronage
+rose 24→41 visits** (peds now reach shops across the connected graph). `fragmentPedFraction: 0`. The sim
+needed nothing — exactly as R18 predicted (a single component is strictly easier than 105).
+
+### Handshake
+- **D → F:** fragmentation verdict **post-fix is GREEN** — real Katoomba is one connected component, 0 peds
+  stranded, 0 NaN, all citizen behaviours hold, determinism intact (A's golden pins it). No D-side gate
+  concern. Tag away.
+- **FYI (B/F budget):** edges rose 799→966 (junction protection + bridges) though total street-metres fell
+  (islands culled) — flagged for the tris budget alongside the BIG_CITY fix; not a sim concern.
+- No D-side spawn-bias mitigation needed this epoch — A's pass left nothing to mitigate. (Beta backlog item
+  retired.)
+
+---
+
 ## ROUND 18 — v4.0-alpha REAL ROADS: citizens on real bones (ledger #5)
 
 *PROCITY-D, 2026-07-16. The audit: does the citizen sim behave on real (irregular) street topology?