testdata: expand the schema-extensions conformance corpus (#68)

[trendvidia]

Closes #68. Expands the corpus from 12 to 20 numbered fixtures plus an invalid/ suite, covering all five bullets of the issue:

Declaration shapes — 13_declaration_shapes.proto. Paren-less and empty-paren annotation declarations; params of every §5.1 type (string/int32/int64/float/double/bool/bytes/any/expression and qualified-ident enum + message types) with defaults incl. negative literals; zero-param and multi-param functions; a function with both an option list and trailing annotations; a bare type alias; use sites with no parens, empty parens, all-defaulted args, positional-then-named args, bytes/float/negative args, bare enum-value refs, and message literals with and without the optional leading type name.

Composition — 14_refinement_kinds.proto. Enum, wrapper (plus a chained derived alias), and message refinement; field-level rules stacked atop a chain; an alias as a repeated element type. (Chained refinement over primitives was already covered by 02.)

Placements were already covered by 03 (message/oneof/service/rpc/enum/enum-value leading, field/enum-value trailing); 13 adds the trailing annotation list on functionDecl.

Error states — one report-golden fixture each, following the 07 executable pattern (schema + instance directory, top-level golden, pinned stubs/catalogs in the schema header):

• 17_missing_impl_golden — missing impl: lenient engine ⇒ reserved protowire.function.unimplemented with the spec-pinned fallback template; strict engines fail startup.
• invalid/arity_mismatch.proto — invalid signature: declared-function call arity mismatch is a compile error (§8.1).
• 18_default_unsatisfiable_golden — unsatisfiable rule: @default failing its own field's rules ⇒ RULE_KIND_DEFAULT with the substituted default as actual_value.
• 19_catalog_miss — locale catalog miss: Report golden plus a messages_de.txt rendering golden (catalog hit interpolates function-authored params; miss falls back verbatim).

Plus two goldens the ledger routed to #68: 15_collections_golden (per-element repeated/map validation incl. the key dimension with for_key, #141/#153) and 16_sensitive_golden (§6.7 redaction at all three attachment sites, #90/#21).

Round trip. Documented in the README as the corpus's descriptor-level contract and verified here: the full positive corpus (all 17 schema fixtures + libraries, 24 files) compiles and lowers through the reference protocompile pipeline, and the lowered FileDescriptorSet round-trips through stock protoc (decode → re-encode with the carrier + pxf schemas in scope) byte-identically (§8.5). All six report goldens, the config golden, and the carrier golden protoc --encode cleanly. The executable harness stays in protocheck's roundtrip/ (private-module graph); expanding its fixture list rides the next protowire pin bump.

Verification. Every positive fixture compiled + carrier-dumped through local protocompile; all eight invalid/ fixtures rejected with diagnostics attributable to their manifest error class; goldens 16–19 are byte-for-byte the reports protocheck v2.1.0 emits (engine/wall_time omitted per cross-port equality); golden 15 matches except the map-key violation, which is spec-authored because the engine does not validate the key dimension yet.

Reference-toolchain gaps found and filed while building this:

• engine: map keys are not validated against the key type's rules (§6.4 / protowire#141) protocheck#53 — engine skips map-key validation (golden 15 leads it).
• fdp: TypeDecl.base_type_fqn is not fully qualified for in-package bases protocompile#121 — TypeDecl.base_type_fqn lowers as-written instead of fully qualified (fixture 14 pins the FQN expectation).

Ledger #19 marked resolved in docs/RFC-001-issues.md.