[recipe] OTTL wire-up for tracecore.alert.training_step_stalled (pattern #7)

[trilamsr]

Pattern-7 detector consumes tracecore.alert.training_step_stalled.no_progress_seconds bridge attribute. OTTL metrics→logs recipe (RFC-0014 convention, mirroring pattern-5 pcie_rate_collapse shape) is the integration gap.

Scope: OTTL stanza that converts step-duration metric stall into the training-step-stalled log record. Cross-link from docs/patterns/07-dataloader-hang.md.

Tracked from: PR #346 Open Q#1.