docs: phase-0 — OQ-GN-02 direction set: plan-driven contexts

- plans for offsets/sizes (already computed+validated by the engine,
  free at generation time since alktype is already a dep), BAST for
  descriptive metadata; drift risk inverts into ordinary
  dependency-upgrade tracking of alktype's plan API
- residue: exact plan->context projection is the Phase 1 context-
  builder spec task

docs/research/phase-0.md

@@ -163,6 +163,26 @@ offsets/sizes, BAST for descriptive metadata (names, docs), with
 the exact projection decided in Phase 1. This decision shapes the
 whole context-builder layer.
 
+**Direction set 2026-09-08 (setup discussion): plan-driven, with BAST
+for descriptive metadata.** Supporting rationale:
+
+- The layout math (offsets, sizes, dispatch structure) is already
+  computed *and validated* by the engine — re-deriving it in the
+  template layer would reintroduce exactly the drift OQ-GN-03
+  flags as the standing risk, by construction.
+- alktype is already a dependency; plan computation at generation
+  time costs nothing extra.
+- The residual risk inverts: instead of drifting from engine
+  semantics, contexts now track alktype's public plan API. If a
+  plan shape evolves, context builders evolve with it — an
+  ordinary dependency-upgrade concern, not a correctness risk.
+
+Residue for Phase 1 (spec-shape, not research): the exact projection
+— which plan structures (`OffsetMap` entries, `ReadPlan` nodes,
+`ValidationPlan` fields) map to which template-context shapes, and
+where BAST metadata (names, docs, annotations) joins the projection.
+This is the context-builder layer's spec task.
+
 ### OQ-GN-03: Where does the interpreter/compiler duplication get cut?
 
 **Resolved as posture, 2026-09-08 (setup discussion): the duplication
@@ -297,8 +317,10 @@ is a place the compiled path stays interpreter-shaped.
 - [x] Branch scope framed — readers/writers/validators first,
       handlers second, JIT later (§opening)
 - [ ] OQ-GN-01 (output unit) — resolved with alkcall-side input
-- [ ] OQ-GN-02/03 (context source, duplication cut) — the core
-      phase-0 discussion
+- [x] OQ-GN-02/03 (context source, duplication cut) — direction set
+      2026-09-08: plan-driven contexts (plans for offsets/sizes, BAST
+      for descriptive metadata); duplication resolved as posture.
+      Residue: exact plan→context projection (Phase 1 spec task)
 - [ ] OQ-GN-05/06 (delegation boundary, dependency shape) — the
       reader-POC cluster
 - [ ] OQ-GN-07 (validation strategy) — resolve with or after