docs(conventions): Method._missing_ folds case against RFC 9110 §9.1, and registry-protocol.rst records neither the fold nor the get/constructor split #2552

From the 2026-11-01 conventions audit, re-verified on 73add05a3. registry-protocol.rst, "RFC-Directed Case Sensitivity": a case fold has to cite the RFC or IANA registry that makes the values case-insensitive, else it is a defect. The Method row says get does not fold (#896); the note on a fold outside any get names only esp._resolve.

• Method._missing_ in pcapkit/const/http/method.py upper-cases value and matches member names, then values: Method('get') is Method.GET, while Method.get('get') returns an unregistered member named GET with value 'get'. RFC 9110 §9.1 makes the method token case-sensitive.
• Its docstring calls the split deliberate and cites fix(const): Method.get is case-insensitive, but RFC 9110 makes the method token case-sensitive #896, but fix(const): Method.get is case-insensitive, but RFC 9110 makes the method token case-sensitive #896's thread concerns get alone (the fix removed the fold from get); fix(const): Method('baseline-control') mints an unregistered member, not BASELINE_CONTROL #2284 and fix(const): find hyphenated HTTP methods by value in Method._missing_ (#2284) #2351 later widened the constructor path. The rule recorded on design: should the non-registry helper enums share a lookup interface, or be made strictly immutable? #877 lets a registry fold only where an RFC recognises it.
• Stale wording on the same page: Method.get "catches it in order to mint". It returns _unregistered_member(...), which grows neither lookup table, and mint-criterion.rst defines minting as extend_enum.
• Probe: git show origin/main:pcapkit/const/http/method.py | grep -n "value.upper()".

Decision needed: does the constructor fold stay?

1. Keep it: record the get/constructor split on the page with its basis, and correct the docstring's attribution.
2. Drop it: remove the fold from the _missing_ template in pcapkit/vendor/http/method.py and regenerate (observable: Method('get') stops resolving to GET).
   Recommendation: 1, because fix(const): Method('baseline-control') mints an unregistered member, not BASELINE_CONTROL #2284 asked for Method('baseline-control') to resolve to BASELINE_CONTROL and fix(const): find hyphenated HTTP methods by value in Method._missing_ (#2284) #2351 did it through this fold; dropping it reverses that.
   Fix: reword "to mint" either way; option 1 edits registry-protocol.rst, which feat(registry): foundation.registry wrappers for IPX and SCTP sub-maps; record the convention (#2511) #2521 also edits.