Lane B: witness purpose taxonomy, derived cost envelope, and derived placement

[gunbai-bot]

Auto-opened by session-dashboard for session royal-tern-308.
Pushing to session/royal-tern-308-witness-purpose-placement advances this PR.

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[gunbai-bot]

Closing as superseded by #7893, after a declaration-grain comparison rather than a file-level one.

A naive check would have said #7893 is simply bigger and closed this immediately. That check is unsafe — a name-level diff is blind to a weaker body — so I compared declarations in both shared files. It initially looked like this PR held unique content: 12 declarations in dag/std/witness_purpose.dag and 4 in dag/std/witness_placement.dag appear here and not in #7893.

They are not lost capability. They are the pre-relocation spellings. #7893 moved the envelope onto the existing scheduling authority instead of minting a parallel one in witness_purpose, which is the DESIGN §3 single-authority move and is why the names moved:

this PR (#7890)	#7893
WitnessCostEnvelope	ScheduledWitnessEnvelope (in std.realization_schedule)
WitnessCostEnvelopeVerdict	WitnessEnvelopeCompareVerdict
witness_cost_envelope_exceeds_fast_lane	envelope_exceeds_fast_lane
witness_cost_envelope_admits_observed_wall	envelope-compare verdict machinery
WitnessResourceAxis	WitnessResourceBound
WitnessRequiredCadence	WitnessEnvelopeCadenceWire + derive_witness_route_scheduled_cadence
WitnessDisposition	OnSuccessRunnableDisposition
witness_route_is_per_pr, witness_route_has_executing_consumer, witness_consumer_cadence_executes	WitnessRoute + derive_witness_route family
witness_disposition_terminating_survey_note	witness_purpose_terminating_one_shot_note

#7893 additionally carries the scheduling integration, the Rust realization, and a witness file of 531 lines against this PR's 231.

Why closing rather than leaving both open: two open PRs defining the same purpose/placement vocabulary are two candidate authorities for one concept — the nicknaming violation §3 exists to prevent, and it duplicates again in everything derived from it. One has to win, and it should be the one that reuses the existing envelope authority.

If any row above turns out to have no real counterpart once #7893 lands, it should be added to #7893 rather than by reopening this — the mapping is recorded here precisely so that check is possible later.

— sent from proud-bear-834