refactor(stage1-r): rename the strategy-output param namespace exposure -> bias

Complete the deferred rename tail from cycle 0065's #126 (which renamed only the
typed metric, keeping the param namespace uniformly "exposure" so the single-run
rename would not smuggle in a ~30-site sweep-pinned change). Now renamed together
as one consistent unit, all driven by the Bias node's instance name:

- Bias::builder().named("exposure") -> .named("bias") (the harness builders +
  the engine fixtures in test_fixtures.rs / blueprint.rs);
- the derived knob path exposure.scale -> bias.scale (Composite::param_space
  derives the knob from the node name) -- the sweep .axis/.range bindings and
  .with() points (blueprint.rs, main.rs), the walkforward ParamSpecs
  (walkforward.rs), the member_key dir names, and the JSON byte-pins in
  cli_run.rs / report.rs / main.rs tests;
- the exposure_scale manifest param label -> bias_scale (the single-run manifest
  builders + the streaming_seam / real_bars fixtures).

The rename is surgical: the three targets (named("exposure"), exposure.scale,
exposure_scale) are syntactically distinct from the deliberately-kept "exposure"
forms, which are untouched -- SimBroker's pre-reframe exposure input slot +
prev_exposure + the exposure-integral; the LongOnly / gate / latch ports named
"exposure"; and the on-disk "exposure" tap label (the recorded bias series that
feeds `chart --tap exposure`, decoupled from the node name via
ColumnarTrace::from_rows("exposure", ...)).

No on-disk back-compat break: the knob path is a runtime param address, and
recorded params in old runs.jsonl / families.jsonl are inert data strings (a
pre-existing recorded "exposure.scale" still loads, it is just data).

INDEX.md cycle-0065 realization note amended: the param namespace is recorded as
the now-completed tail; the SimBroker slot + the on-disk tap label remain the
deliberate permanent keeps (#117), and exposure_sign_flips stays a serde alias.

Verified: cargo build --workspace --all-targets clean; clippy --all-targets
-D warnings clean; full suite 514 passed / 0 failed (source + byte-pins renamed
together); live `aura run` emits "bias_scale", `aura sweep` emits "bias.scale".

closes #134
refs #126 #117

crates/aura-ingest/tests/real_bars.rs

@@ -84,7 +84,7 @@ fn run_sample_over(prices: Vec<(Timestamp, Scalar)>) -> RunReport {
             params: vec![
                 ("sma_fast".to_string(), Scalar::i64(2)),
                 ("sma_slow".to_string(), Scalar::i64(4)),
-                ("exposure_scale".to_string(), Scalar::f64(0.5)),
+                ("bias_scale".to_string(), Scalar::f64(0.5)),
             ],
             window,
             seed: 0,

crates/aura-ingest/tests/streaming_seam.rs

@@ -84,7 +84,7 @@ fn run_sample(window: (Timestamp, Timestamp), source: Box<dyn Source>) -> RunRep
             params: vec![
                 ("sma_fast".to_string(), Scalar::i64(2)),
                 ("sma_slow".to_string(), Scalar::i64(4)),
-                ("exposure_scale".to_string(), Scalar::f64(0.5)),
+                ("bias_scale".to_string(), Scalar::f64(0.5)),
             ],
             window,
             seed: 0,