Three classes of cleanup, prompted by the pre-commit stub-return
heuristic surfacing in the 2c.3c.4 commit:
(1) **Unreachable defensive null/bounds-guards → debug_assert + delete**
`read_isv_signal_at`, `read_atom_utilization`, `compute_q_spectral_gap`
each guarded `is_null()` on `isv_signals_pinned` / `q_readback_pinned`,
yet both pointers are unconditionally allocated by the constructor
(`cuMemAllocHost_v2` with `assert_eq!` on success) and NEVER reassigned
after construction (greppable: zero `self.isv_signals_pinned =` /
`self.q_readback_pinned =` outside Drop). The guards were dead code
masquerading as safety. Removing the early-return removes the ambiguity
between "buffer never initialised" and "buffer holds a real-zero ISV
value" that downstream consumers of `read_isv_signal_at(IDX) == 0.0`
cannot otherwise distinguish.
`read_isv_signal_at`'s `index >= ISV_TOTAL_DIM` early-return is similarly
a stub: every caller passes a named constant from the ISV slot table
(`GAMMA_DIR_EFF_INDEX`, `TAU_EFF_INDEX`, `EPSILON_EFF_INDEX`, ...), so
bounds violation is a programming error not a runtime condition.
`compute_q_spectral_gap`'s `n_cols == 0` early-return is dead for the
same reason — `total_actions()` is the sum of branch sizes which the
trainer config requires non-zero.
All four converted to `debug_assert!(...)` so dev/test builds catch
invariant violations with a clear message; release builds inherit the
UB on the unsafe deref of a null pointer (loud crash) rather than the
silent stub return that previously made misconfiguration look fine.
(2) **Silent training-instability mask → warn-log + collapse sentinel**
`compute_q_spectral_gap` previously returned 1.0 (= "balanced/healthy"
in the calibration's eyes) on the first non-finite Q-value. NaN/Inf
Q-values indicate gradient corruption — masking them with the same
return value as a healthy network defeats HEALTH_DIAG's purpose. New
behaviour: scan all samples, mark the non-finite case, then emit a
`tracing::warn!` and return 100.0 (the same collapse sentinel emitted
when lambda_2 falls below the numerical floor). Operators see the
failure mode in logs rather than chasing a phantom balanced spectral
gap that contradicts NaN losses elsewhere in the same epoch.
(3) **Genuine domain encodings → annotated for the linter**
Three returns in `compute_q_spectral_gap` (lambda_2 < 1e-12, sigma2
< 1e-6) and `power_iteration_largest` (norm < 1e-20) ARE meaningful
mathematical degenerate-case answers, not stubs:
- lambda_2 ≈ 0 / sigma2 ≈ 0: deflated Gram is rank-1, sigma1/sigma2
diverges — calibrated collapse sentinel 100.0.
- norm ≈ 0 in power iteration: M·v vanishes, so the eigenvalue is 0.
Each got an explanatory comment + `// ok: domain encoding` to make
the intent legible to both readers and the heuristic linter.
`power_iteration_largest`'s `n == 0` early-return is unreachable
from the spectral_gap caller (debug_asserted upstream); removed in
favour of `debug_assert!(n > 0)` at the helper entry.
No behaviour change in production runs: the unreachable guards weren't
firing and the new invariants match the constructor's contract. The
non-finite Q-value path now surfaces what was previously silent — a
strict improvement.
cargo check clean at 11 warnings (baseline preserved).
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
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