docs(headless): record K3 isolation checkpoint

docs/research/2026-07-27-slice-k3-shared-content-isolation-checkpoint.md

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+# Slice K3 shared-content and isolation checkpoint
+
+**Date:** 2026-07-27
+**Status:** Automated checkpoint complete; connected Windows/Linux gate remains
+**Commits:** `12b500d3`, `9569dadb`, `b6547ff3`
+
+## Outcome
+
+The production headless process now opens DAT and `acdream.pak` content once,
+leases the exact owner to every configured session, and disposes the mapping
+only after the final lease. Sessions share immutable magic/catalog, terrain
+height, and bounded flat-collision payloads while retaining distinct
+`GameRuntime`, entity, object, action, movement, physics-engine, data-cache,
+scratch, and teardown owners.
+
+The direct inbound route now projects accepted local-player spawn/position and
+portal state into one per-session movement controller and a per-session
+collision neighborhood. Terrain, EnvCell topology, buildings, stabs, and
+retail-generated scenery use the same presentation-free content builder as the
+graphical streaming path. Headless hydration deliberately skips visual bounds,
+so it does not populate the graphical process-wide `GfxObjBounds` cache.
+
+The scheduler quarantines a throwing policy, detaches it before canonical
+teardown deltas, and continues healthy sessions. Headless startup rejects
+process-global physics probes whose mutable capture cursors cannot be
+attributed safely across sessions.
+
+## Automated evidence
+
+- Runtime tests: 412 passed.
+- Headless tests: 42 passed.
+- App tests: 3,722 passed / 3 skipped.
+- Complete Release solution: 8,758 passed / 5 skipped.
+- Streaming/collision focused gate: 277 passed.
+- Parameterized 1/5/10-session gate:
+  - identical server GUIDs remain isolated;
+  - activity and portal completion remain per Runtime root;
+  - reconnect replaces only that session's incarnation/generation;
+  - every teardown ledger converges.
+- Policy-fault gate: one failing policy is quarantined while the healthy
+  session continues ticking.
+- Shared-content gate: exact DAT, package, magic catalog, height-table backing
+  array, and bounded collision source are shared; mutable physics/cache/
+  movement owners are not.
+
+## Remaining K3 gate
+
+1. Run the production Release executable against local ACE in a visible
+   terminal with no window, renderer, audio, or UI assemblies.
+2. Observe initial collision hydration, lifestone recall, reconnect, and exact
+   final content-lease disposal.
+3. Repeat the production gate on Linux/WSL.
+4. Add the connected resource evidence and close K3 before deriving K4's
+   numeric 1/5/10/30-session ceilings.
+
+## Exact rollback
+
+Newest first:
+
+```text
+git revert b6547ff38c375328417855200f41c8af6436f246
+git revert 9569dadb578b299b24e30aec85eee86ec0c2b3fb
+git revert 12b500d38362e8aae0a23a4f9712b86367006f76
+```