[S2] hmm/hmmm metric key inconsistency across 4 files

[erinepshovel-code]

The metric key for hesitation markers is spelled two ways across the codebase:

File	Key used
backend/src/edcmbone/engine.py	"hmm"
backend/src/edcmbone/operator_extractor.py	"hmm" / "hmmm" (mixed)
core/bridge/bridge_engine.py	"hmm"
core/behavioral/behavioral_metrics.py	"hmm"

Where a writer uses "hmmm" and a reader uses "hmm" (or vice versa), the hesitation signal is silently dropped — the lookup returns 0.0 or None with no error. Bridge correlations involving the hesitation metric are computed from a flat-zero input on the mismatched side.

Fix: Canonicalize to one spelling (suggest "hmm") across all four files. Define a shared constant:

# in a shared constants module
HMM_KEY = "hmm"

and import it at each of the four sites to eliminate the risk of future re-divergence.

[erinepshovel-code]

Fifth file: tests/run_all.py ends with print("hmm:") — using the 2-m spelling. Updated file count: 5 files total (engine.py, operator_extractor.py, bridge_engine.py, behavioral_metrics.py, run_all.py).

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