FABRIC.md §25: item 1.3 inherits §18.5's determinism constraint

Review item REVIEW-D3.

Item 1.3 decides what triggers a capacity transfer and said it should read the
density gradient rather than a schedule -- correct about what, silent about
when. Hera's arbitration mutates patron mass, so §18.5 binds it directly:
anything influencing patron state advances on tick count, and wall-clock time
must never be an input to a decision.

Without that stated in the item, a reasonable implementation could pace
arbitration off a wall-clock interval and reintroduce, in a new place, exactly
the defect item 2.1 exists to remove. Whatever 1.3 decides must be expressible
in ticks.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

FABRIC.md

@@ -2057,7 +2057,13 @@ document and committing that amendment as its own item.*
   mass of its pinned patrons (derived) or a constant (tuned). *Refs:* §22.5 #1.
 
 - [ ] **1.3 — What triggers a capacity transfer.** Hera arbitrates; on what signal, and how
-  often. Should read the density gradient, not a schedule. *Refs:* §22.5 #2.
+  often. Should read the density gradient, not a schedule.
+  **Constraint, not optional:** arbitration mutates patron mass, so §18.5's invariant binds
+  it directly — *anything that influences patron state advances on tick count; wall-clock
+  time may be recorded for diagnostics and must never be an input to a decision.* Pacing
+  arbitration off a wall-clock interval would reintroduce exactly the defect item 2.1
+  exists to remove, in a new place. Whatever 1.3 decides must be expressible in ticks.
+  *Refs:* §22.5 #2, **§18.5**, §22.4.
 
 - [ ] **1.4 — The heat/capacity timescale ratio.** The ordering is fixed (capacity slower);
   the ratio is not. *Refs:* §22.4, §22.5 #3.