docs: update architecture gate progress

BUILD_IN_PUBLIC_PLAN.md:
- Mark systems gate items deterministic clock and headless checksum
  as complete; remaining gate work stays flagged as in-progress
- Rename TaskZone references to ActivityMarkers for consistency

LEARNING_ROADMAP.md:
- Add architecture gate progress note: first deterministic slice
  (SimulationClock, seeded streams, headless checksum) is implemented
- Update recommended implementation order to reflect shifted priorities

PROJECT_CONTEXT.md:
- Note that farm and forest markers have been deleted from main.tscn
- Update activity marker description (4 remaining: guard/study/rest/food)
- Add SimulationClock.gd and deterministic_simulation_test.gd to repo map
- Update technical debt: randomness is now seeded but RNG state is not
  yet serialized; clock is explicit but orchestration still lives on scene tree
- Gate status updated with completed first-slice summary

RESOURCE_NODE_MIGRATION.md:
- Mark Phase 4b (player parity) complete
- Phase 6 renamed from 'remove zone model' to 'replace activity markers'
  with progress note: TaskZone container and FarmZone/ForestZone deleted
- Update divergences to note farm/forest scene nodes have been removed

docs/LEARNING_ROADMAP.md

@@ -54,6 +54,18 @@ The gate is complete when the same scenario produces the same checksum without
 loading `main.tscn`, and loading or unloading a visual does not change
 authoritative NPC or resource state.
 
+**Current progress:** the first deterministic slice is implemented:
+
+- `SimulationClock` advances explicit fixed ticks while preserving remainder;
+- NPC decisions and visual wander requests use controlled per-NPC random
+  streams derived from an exported simulation seed;
+- a headless scenario verifies same-seed equality, different-seed divergence,
+  and a final-state checksum without loading `main.tscn`.
+
+The gate remains open. Versioned state records, scene-independent resource
+authority, stable definitions, responsibility separation, and visual
+load/unload invariance are still required.
+
 ## Three vertical slices
 
 The long-term vision contains three major risks. Validate them separately before
@@ -599,8 +611,8 @@ Completed foundations:
 
 The practical next sequence is:
 
-1. Begin the architecture gate with deterministic clock/randomness, a headless
-   scenario runner, final-state checksums, and serializable state records.
+1. Add versioned serializable NPC, village, and resource state records,
+   including enough clock and RNG state for deterministic continuation.
 2. Move resource amount/reservation authority out of `ResourceNode` scenes.
 3. Replace free-form task and profession strings with stable IDs and
    definitions.