chore: add a petstore codegen spike as the witness for the codegen target surface (#64)

[Wahbeh-Mohammad]

Closes #64.

examples/petstore/ is a throwaway spike, not a shipped example: a frozen OpenAPI document byte-identical to the Python witness's, a deterministic generator, a projection-only facade, a hand-written ServiceCore, a regen-diff guard, and a 15-case canary over an in-memory transport covering unary calls, cursor pagination, SSE with a [DONE] sentinel, a merge-patch body's three Tristate states, all three auth-tier outcomes, and both close semantics. The deliverable is examples/petstore/FINDINGS.md, which confirms all five hypothesised gaps — no executor tier, no StatusErrorMap, OperationDescriptor merging its static and per-call halves, the operation auth tier having no source, and FakeTransport being unreachable from the entry point — and records five more found while building, including that Schema<T> is decode-only and that Paginator has no status-mapping hook. It also corrects the plan's isolation claim: four gates ignore examples/, but gts lint . does not, which is why the example carries its own tsconfig.json for eslint's projectService and why the generator formats its output through the same gts/.prettierrc.json the lint rule uses. Nothing under packages/, tests/, scripts/, bunfig.toml, package.json or eslint.config.js is touched, no package depends on the tree, and bun run test collects 164 files before and after — so no changeset, since nothing consumer-facing changed.

Verified with run-ci.mjs --clean --node-floor (19/19 green, Bun pinned to 1.3.14, test:node also on Node 20.3.0), a clean housekeeping probe, and bun test ./examples/petstore at 15/15.