feat(pathfinding): non-eager TryGetMask + second-touch promotion

[kamronbatman]

Summary

Closes the Cold-cache regression flagged in PR #2450. StepCache.TryGetMask no longer eagerly runs BuildChunk on the first miss for a chunk that isn't in a .swb lazy reader. Instead it returns Fallthrough_NotBuilt and the caller (BitmapAStarAlgorithm) takes the per-cell slow path. The chunk is only promoted to the bitmap fast path after the second miss within a 30-second window, filtering single-touch pass-throughs.

This makes BitmapAStar's worst-case (cold cache + short hops) collapse from 12–47× slower than FastAStar to roughly the same, which is the floor the slow path can deliver. Steady-state warm performance (the actual deliverable) is unchanged from PR-5 — it was always the cache fast path.

The pet-follow scenario this fixes

A mounted player at ~4 tiles/sec with a pet/hireable following will trigger an NPC pathfind every 100–300 ms. Each pathfind is 1–6 tiles. As the player crosses chunk boundaries (~4 sec/chunk), the pet's first pathfind in the new chunk under the previous behavior triggered a full ~700 µs BuildChunk for a chunk the player would leave shortly after. At 50–100 mobiles per shard, this exceeded the 8 ms tick budget. PR-5 BDN data showed scenarios 6–9 (2–8 tile NPC perception) at 2,300–3,700 µs Cold vs FastAStar's 80–200 µs.

Under the new gate:

• First miss → Fallthrough_NotBuilt → caller uses slow path (~30–50 µs short path). No BuildChunk. No allocation.
• Player keeps moving → chunk never gets a second touch within window → never promoted, no rot.
• NPC patrolling a fixed territory → repeatedly hits the same chunks → second touch within window → promote → cache fast path on subsequent calls.

What changed

• CacheHitKind.Fallthrough_NotBuilt = 6 + CacheStats.FallthroughNotBuilt counter. IsHit=false, so the caller routes to slow path.
• StepCache._chunkMissTracker — Dictionary<long, ChunkMissState> capped at 4096 entries. State is (byte missCount, uint lastMissTickStamp) keyed by chunk key. Window-expired entries reset count to 1; capacity overflow prunes window-old entries first.
• StepCache.MissPromotionThreshold (default 2) and StepCache.MissPromotionWindowMs (default 30_000) — tunable, can be wired through ServerConfiguration if shards want different policy. Setting threshold to 1 restores legacy eager-build behavior (used by tests that prime chunks via single TryGetMask call).
• StepCache.TryGetMask miss branch — try lazy reader first (file-loaded chunks bypass the tracker entirely; an .swb represents an explicit prior decision to keep the chunk warm). Otherwise consult the tracker.
• BitmapAStarAlgorithm.GetSuccessorsSlowPath now layers IsBlockedByDynamic on top of CalcMoves.CheckMovement. Previously the slow path only ran for CanFly creatures and rare cache fallthroughs — CheckMovement doesn't iterate same-cell mobiles, so the bitmap fast path's IsBlockedByDynamic was the only mobile-blocking check. Now first-touch pathfinds run through the slow path, so the gap had to close.

Tests

50 pathfinding tests pass (was 47). New / updated:

• TryGetMask_FirstTouchOnUnbuiltChunk_DefersBuildAndReturnsFallthrough — single TryGetMask call returns Fallthrough_NotBuilt, no chunk built, no allocation.
• TryGetMask_SecondTouchWithinWindow_PromotesAndBuilds — second call inside the 30s window builds + serves.
• TryGetMask_SecondTouchAfterWindow_RestartsCounterAndDefers — second call outside the window restarts the count, returns Fallthrough again.
• TryGetMask_DistinctChunks_TrackedIndependently — counters are per-chunk; one touch on each of two adjacent chunks both stay in fallthrough.
• LazyReaderHit_BypassesMissTrackerOnFirstTouch — open .swb + first touch hits without consulting the tracker. Production with .swb loaded skips the gate entirely.
• MultisVersion_Bump_TriggersDirtyRebuild — updated to reflect the new 3-step flow (Fallthrough → Miss_NotBuilt → Miss_DirtyRebuild).
• Tests that prime chunks via a single TryGetMask call (multi-Z, Tier4, lifecycle, parity, BitmapAStar uses-cache) set MissPromotionThreshold = 1 to opt into eager behavior.

Expected BDN impact

The Cold column from PR-5's BDN should change as follows once the bench's submodule pointer is updated to this branch:

#	Scenario	Cold (PR-5)	Cold (PR-6 expected)	FastAStar Cold
2	sewer corridor	1,627 µs	~36 µs	36 µs
4	causeway	1,533 µs	~39 µs	39 µs
6	pet 2-tile	2,364 µs	~80 µs	81 µs
8	npc 5-tile	3,708 µs	~140 µs	141 µs
9	npc 8-tile	2,386 µs	~200 µs	197 µs

WarmNoFile and LazyWarm rows should be unchanged — they were always cache-warm. The miss tracker only fires when neither resident chunks nor the lazy reader can satisfy the request.

Future work (not in this PR)

• Background-thread bake: builds outside the game thread so even promoted chunks don't pay the 700 µs build cost on the main thread. Rule 10 (no Task.Run) applies, so this needs careful design — the bake is a pure data transform but main-thread synchronization on chunk-state transitions has to be threaded through. Defer to a follow-up.
• Long-traverse BDN scenario: a multi-Find benchmark simulating 50 pet repaths across chunk transitions. Requires restructuring the bench harness; the existing 10-scenario corpus + Cold provider already exercises the gate.
• Swim sourceZ bake: scenario 5 (sea serpent) shows 56 B alloc on warm paths because the cache's SourceZ is computed under default-walker rules. Swim creatures fall through to slow path. Independent of this PR.