plan(dqn-v2): Plan 3 self-review fix — remove unimplemented!() from example

Self-review caught that the CqlAlphaSeedCoupledController read_signals
example used unimplemented!("plumbed by caller"), which violates the very
Invariant 9 the plan enforces. Replaced with a concrete implementation
that reads SEED_FRACTION_EMA_INDEX from the ISV bus (a Plan 1 reserved
controller-signal slot).

No other plan had placeholder violations. The remaining TBD/TODO/FIXME
grep hits across Plans 1-5 are either the pre-commit hook's rejection
patterns (correctly quoted) or enforcement scans that DETECT the markers
in audit docs.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

docs/superpowers/plans/2026-04-24-dqn-v2-plan-3-behavioural.md

@@ -1297,10 +1297,12 @@ impl AdaptiveController for CqlAlphaSeedCoupledController {
     type Signal = f32;  // current seed fraction in replay buffer
     type Control = f32; // cql_alpha
 
-    fn read_signals(&self, _isv: &IsvBus) -> Self::Signal {
-        // Producer: replay buffer tracks seed-origin vs. dqn-origin count.
-        // That's exposed via an existing counter; controller reads it.
-        unimplemented!("plumbed by caller")
+    fn read_signals(&self, isv: &IsvBus) -> Self::Signal {
+        // Producer: replay buffer tracks seed-origin vs. dqn-origin count
+        // and writes the current seed fraction into ISV slot
+        // SEED_FRACTION_EMA_INDEX (a Plan 1 reserved controller-signal slot).
+        // Here we just read it back.
+        isv.read(SEED_FRACTION_EMA_INDEX)
     }
 
     fn update(&mut self, seed_fraction: f32) -> f32 {