fix(camera): #38 render-interpolate player motion

Keep local physics authoritative at the retail 30 Hz MinQuantum, but expose a render-only position that lerps between completed physics ticks for the player mesh and chase-camera target. Network outbound continues to use the discrete physics position.

Also make the visually confirmed #47 humanoid close-detail DIDDegrade path default-on, with ACDREAM_RETAIL_CLOSE_DEGRADES=0 left as a diagnostic opt-out.

Verification: dotnet build AcDream.slnx -c Debug; focused #38 interpolation tests passed; visual confirmed smooth 2026-05-06. Full dotnet test AcDream.slnx -c Debug --no-build still has the known 8 AcDream.Core.Tests baseline failures.

Co-authored-by: Codex <codex@openai.com>

docs/research/2026-05-06-issue-38-render-interp-pseudocode.md

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+# Issue #38 render interpolation pseudocode
+
+## Problem
+
+Phase L.5 correctly restored retail's `CPhysicsObj::update_object`
+physics gate: the body only integrates when accumulated frame time reaches
+`PhysicsBody.MinQuantum` (1/30 second). That keeps collision behavior aligned
+with retail, but the renderer currently reads `_body.Position` directly.
+
+At 60+ FPS that means several rendered frames can show the same physics
+position, then jump to the next 30 Hz position. Chase camera makes the stepping
+obvious because both the player mesh and camera target follow that discrete
+physics sample.
+
+## Evidence
+
+- Named retail: `CPhysicsObj::update_object` at `0x00515d10`
+  (`docs/research/named-retail/acclient_2013_pseudo_c.txt:283950`) skips or
+  consumes time outside the valid quantum window and calls
+  `CPhysicsObj::UpdateObjectInternal` only for accepted physics quanta.
+- ACE mirrors the same constants in `PhysicsGlobals`: `MinQuantum = 1/30`,
+  `MaxQuantum = 0.1`, `HugeQuantum = 2.0`.
+- ACE's `InterpolationManager` / `PositionManager` prove client-style
+  smoothing is a known AC-family concept, but their queue chases network target
+  positions. Issue #38 is narrower: local render-only interpolation between the
+  last two authoritative local physics ticks.
+- Glenn Fiedler's "Fix Your Timestep!" describes the canonical fixed-timestep
+  accumulator plus render interpolation pattern:
+  https://gafferongames.com/post/fix_your_timestep/
+
+## Intentional divergence
+
+Retail did not need a separate high-FPS render interpolation layer because the
+2013 client effectively presented near the same cadence as its physics gate.
+acdream renders at modern display rates, so we preserve retail's 30 Hz physics
+truth but draw a blended visual position between the last two physics truths.
+
+## Pseudocode
+
+State stored by `PlayerMovementController`:
+
+```text
+physicsAccum: leftover render time not yet consumed by physics
+prevPhysicsPos: body position at the start of the most recently completed tick
+currPhysicsPos: body position at the end of the most recently completed tick
+```
+
+On spawn, teleport, or any authoritative SetPosition:
+
+```text
+body.Position = newPosition
+prevPhysicsPos = newPosition
+currPhysicsPos = newPosition
+physicsAccum = 0
+```
+
+Each Update(dt):
+
+```text
+physicsAccum += dt
+
+if physicsAccum > HugeQuantum:
+    physicsAccum = 0
+    prevPhysicsPos = body.Position
+    currPhysicsPos = body.Position
+    return/render body.Position
+
+if physicsAccum >= MinQuantum:
+    oldTickEnd = currPhysicsPos
+    preIntegratePos = body.Position
+
+    tickDt = min(physicsAccum, MaxQuantum)
+    calc_acceleration()
+    UpdatePhysicsInternal(tickDt)
+
+    postIntegratePos = body.Position
+    resolve collision from preIntegratePos to postIntegratePos
+    body.Position = resolved physics position
+    update contact/cell/velocity exactly as before
+
+    prevPhysicsPos = oldTickEnd
+    currPhysicsPos = body.Position
+    physicsAccum -= tickDt
+
+alpha = clamp(physicsAccum / MinQuantum, 0, 1)
+renderPosition = lerp(prevPhysicsPos, currPhysicsPos, alpha)
+```
+
+Important constraints:
+
+- Never write `renderPosition` back to `_body.Position`.
+- `MovementResult.Position` remains the authoritative physics/collision/network
+  position.
+- Add `MovementResult.RenderPosition` for mesh and camera drawing only.
+- Outbound `MoveToState` / `AutonomousPosition` keep using
+  `MovementResult.Position`.
+- If no physics tick has completed yet, `prevPhysicsPos == currPhysicsPos`, so
+  interpolation is a no-op.