fix: fs-aware envelope band default and clean refusal on undeclared unit (v0.9.1)

[LGDiMaggio]

Summary

Patch release v0.9.1. Two robustness fixes of the same class, found by diagnosing crash reports that originated on a pre-0.9.0 server. Both reported symptoms were already resolved in v0.9.0 at the tool boundary; this release hardens the underlying functions so the same failures cannot resurface through any other caller.

Fixes

generate_envelope_report — fs-aware default band
The default upper band edge was a fixed 5000 Hz, which exceeds Nyquist on any signal sampled below 10 kHz (e.g. 8 kHz), so a default envelope report on such a signal raised an invalid-band error. The default now resolves fs-aware to min(5000, Nyquist-1) — matching the digital-filter corner realized downstream and compute_envelope_spectrum's own default. An explicitly requested band above Nyquist is still rejected (never clamped silently). Reproduced at fs=8000 and fs=5000; fixed.

ISO unit conversion — clean refusal on an undeclared unit
_convert_to_velocity_mm_s / assess_severity_raw crashed with 'NoneType' object has no attribute 'lower' when handed a None (undeclared) unit. The live diagnose_vibration / assess_severity paths already guard this (a structured refusal), so the crash was not reachable through the tools — but the pure conversion now refuses None with an actionable message too, hardening the latent crash for any direct caller. It refuses, it never guesses a unit (consistent with the v0.9.0 declared-unit discipline). Reproduced directly; fixed.

Note: the reported "default to 'g'" idea was intentionally not taken — defaulting a unit reintroduces exactly the amplitude-based guessing that v0.9.0 removed.

Tests & verification

• Test-first: both bugs reproduced (opaque AttributeError; invalid-band ValueError at fs=8000) before the fix, then confirmed fixed. Added a regression for each, plus a guard that an explicit over-Nyquist band still raises.
• Tool inventory fixture regenerated for the filter_high signature change (float=5000 → Optional[float]=None); the surface stays 33 tools + 3 prompts.
• Full suite: 887 passed, 17 skipped, coverage 92.18%.

Fixes the crashes tracked while running /ce-debug.

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