refactor: name the vendor OpenAPI specs for what they are

[ShocOne]

Stacked on #112 — base is chore/repin-thousandeyes, so the diff shows only the rename. Merge #112 first and this retargets to main cleanly.

"Corpus" named a collection without saying whose documents it held or what they were for, and it sat one letter of context away from spec — which in this repo already means the single document a provider is generated from (internal/spec/*, spec/upstream.yaml, spec/revised.yaml, tfpfgen spec import|revise|verify).

These are the opposite of that: third-party documents the toolkit's own tests parse and derive. They are only ever read — never imported, corrected, revised, or generated from.

Was	Now
internal/corpus	internal/vendor_openapi_specs
corpus.lock.json	vendor_openapi_specs.lock.json
TFPFGEN_CORPUS_DIR / _REQUIRED / _LOCK	TFPFGEN_VENDOR_OPENAPI_SPECS_DIR / _REQUIRED / _LOCK
TestUnit_Corpus_*	TestUnit_VendorOpenAPISpecs_*
cache at tfpfgen/corpus	cache at tfpfgen/vendor_openapi_specs

The directory takes underscores, following intermediate_representation, the other fully-worded package here. Go identifiers spell the acronym OpenAPI, as the other 74 occurrences in the tree do — OpenApi appears nowhere.

docs/glossary.md gains the term and records corpus as retired; CLAUDE.md's hard rule is restated in the new vocabulary, with the reason a pin exists at all added to it.

Mechanical throughout. No behaviour changes, and the pins themselves are untouched.

Verification

Cold cache, corpus— sorry, vendor-spec tests forced to run rather than skip:

$ TFPFGEN_VENDOR_OPENAPI_SPECS_DIR=$(mktemp -d) TFPFGEN_VENDOR_OPENAPI_SPECS_REQUIRED=1 \
    go test -race -covermode=atomic ./internal/...
all packages pass

$ bash scripts/coverage_gate.sh    91.3% total (vendor_openapi_specs 90.7%)
$ bash scripts/repo_hygiene_gate.sh   pass
$ golangci-lint run                0 issues
$ gofmt -l .                       clean

The renamed env var is proven wired rather than silently ignored — pointing it at an unwritable path fails as it should:

$ TFPFGEN_VENDOR_OPENAPI_SPECS_DIR=/dev/null/nope … go test ./internal/specmodel
    creating /dev/null/nope/openapi: mkdir /dev/null: not a directory

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