docs(decisions): address PR #129 review — D-191 §8 amendment + boundary notes

Blocking PR #129 items 1, 2 (plus polish 16 and 17):

- D-191 §8 prose rewritten to match the code. The previous amendment said
  positions were `{x, y}` objects against a "typically 1024 × 512" grid,
  but the generator, the six hand-authored templates, and all 2394
  procedural seed files ship `[row, col]` integer arrays against a
  `{"w": 512, "h": 256}` grid. The decision doc is now aligned with
  reality: positions are `[row, col]`, the storage grid is 512 × 256,
  and the row-first ordering is called out explicitly so readers can
  cross-reference NumPy/flood-fill/A*/cost-grid conventions.

- §8 now follows the D-094 amendment pattern. The superseded 2026-04-10
  prose is preserved verbatim as "Original (superseded)" with a dated
  Amendment block on top — future readers can see what changed and why
  instead of silently losing the history.

- brands.toml header gains a short Phase 2 boundary note. The 4 anchor
  brands come from D-189 §5; the additional ~23 brands from D-189 §11
  are deliberately deferred to Phase 3 — Phase 2 only needs the demand-
  node plumbing and V-B01..V-B06 validation exercised end-to-end.

- systems-schema.sql `bodies.terrain_reference` comment now pins the
  repo-root-relative path convention (wiki/star-systems/<slug>/bodies/
  <body_id>/heightmap.png) so the three downstream pipelines (populate,
  atlas generator, client loader) share a documented contract instead
  of drifting against an unwritten convention.

wiki/economics/corporations/brands.toml

@@ -7,6 +7,13 @@
 # tâtonnement simulation (D-185: brands consume commodities, not the reverse).
 # Pricing model and cultural premium curves are Phase 3+ deliverables (D-189).
 #
+# Phase 2 boundary: only the 4 anchor brands from D-189 §5 are authored here
+# (Calloway, VGV, thrds, Bífröst Marmor). The additional ~23 brand corps
+# listed in D-189 §11 are deliberately deferred to Phase 3 — Phase 2 only
+# needs the demand-node plumbing + V-B01..V-B06 validation to be exercised
+# end-to-end. Add new brands to this file; the importer and validators pick
+# them up without schema changes.
+#
 # Fields per [[brand_products]] entry:
 #   brand_product_id     Unique slug (corp_id + product slug)
 #   corp_id              Must match a slug in wiki/corporations/