#698. Centralizes the rpc.discover host-side validator
that today lives ad-hoc on the FastLED Python side. Folded into
fbuild-serial (per the project's 'never add a new crate' essential
rule) since it sits next to messages.rs's JSON-RPC types and the
bring-up orchestrator (#697) is the canonical consumer.

What ships:

- RpcSchema + MethodDecl — typed shape of rpc.discover's response
- parse_schema_response(&str) -> Result<RpcSchema, SchemaParseError>
  — distinguishes InvalidJson, MissingJsonRpc, MissingMethods,
  DeviceError variants so callers get actionable failures instead of
  the cryptic '2 validation errors for RpcSchema: jsonrpc required,
  methods required' pydantic message FastLED's autoresearch emits
  today (FastLED/FastLED#3339)
- Accepts both canonical envelope shape (result wrapper) AND bare
  schema shape (no envelope) — different firmwares emit different
  shapes; pin both
- ValidationReport + MethodStatus { Present { params, returns } |
  Missing } — structured per-method status with the device's
  signature surfaced when bound
- validate_against_expected(&schema, expected) — builds the report;
  is_passing() iff every expected method is Present
- Display renders the actionable diagnostic the issue specifies:
  'Expected method 'echo' bound — present (signature: int(int))'
  / 'Expected method 'ghost' bound — NOT FOUND in device schema.'
  / 'Device returned N methods total; harness expects M.'

12 unit tests covering: canonical envelope + bare schema parses,
MissingJsonRpc / MissingMethods / DeviceError / InvalidJson typed
errors, all-present passing report, missing-method failing report,
device-method-count surfacing for extras, actionable Display for
both pass + fail, empty-expected trivial pass.

Out of scope (follow-up):
- fetch_schema() doing the actual serial round-trip — lives in
  the bring-up orchestrator (#697) where the connection is already
  established
- Strict signature checking (today the report surfaces the device
  signature but doesn't auto-fail; over-fires on int vs int32_t
- FastLED's autoresearch migrated to consume this module

Closes #698.