docs(conventions): small drift on process.rst, mint-criterion.rst and protocol-layer-placement.rst #2549

From the 2026-11-01 conventions audit, re-verified on 73add05a3.

• process.rst, "Issue and Pull Request Labels": the Type table lists const and says each type "corresponds to the subject prefix of the commit". No const: prefix exists and const is additive: refactor(foundation): declare _CODE_DESTINATIONS as a dunder on the enum registries themselves #2403 (refactor,const) and chore(vendor): HIP Group and Packet crawlers write #: in _missing_, not # (#2331) #2381 (chore,const) carry another type, while feat(vendor): add the ARPHRD crawler, DATA-attribute shape (#2495) #2497 and feat(vendor): add the TLS content-type/handshake-type crawlers (#2495) #2503 regenerate pcapkit/const files under feat alone. Probe: gh pr list --state all --label const --json labels.
• mint-criterion.rst, "Minting an Unrecognised Value", the dead-code census: says the str-valued Command, FEATCode and Method "were not probed, so that list is a floor". Each reaches its _unregistered_member call (Command('ZZQX'), FEATCode('ZZQX') and Method('ZZQX') return unregistered members), so the census is complete and the hedge is stale.
• protocol-layer-placement.rst: the headings "IANA Assignments Carry No Layer Claim" and "Tunnelling Protocols Follow Their Payload" carry a finite verb; documentation.rst, "Heading Case and Shape", rewrites such a heading rather than re-casing it.

Fix: move const out of the Type table into a note that it is additive, as breaking is; delete the "floor" sentence; rename the two headings and sweep every reference to them (:ref:, docstrings, tests/, per "Renaming a Heading").