feat: add --enrichments overlay for operationId-keyed metadata

[Arkptz]

Summary

Adds --enrichments <PATH> (alias -e) to mitm2openapi generate. The flag loads a YAML overlay keyed by operationId and applies human-written metadata to the auto-generated spec — after build, before write.

What it does

# enrichments.yaml
info:
  description: Reverse-engineered MEXC web API.

operations:
  getFairPrice:
    summary: Get fair price for a futures contract
    description: Returns the mark price used for liquidation calculations.
    x-requires-auth: false
    x-rate-limit: "10/s"
    x-error-codes:
      - code: 401
        message: Not logged in
    responses:
      "200":
        description: Fair price payload

components:
  schemas:
    ApiError:
      description: MEXC envelope error response.

mitm2openapi generate ... \
  --operation-id-strategy path \
  --enrichments enrichments.yaml

Merge semantics

Scope	Rule
info.*	Overlay wins per-key
operations.<opId>.summary, description, deprecated	Overlay wins
operations.<opId>.tags	Overlay replaces entire list
operations.<opId>.x-*	Passed through verbatim
operations.<opId>.responses.<status>.description	Overlay wins
components.schemas.<name>.description	Overlay wins (properties untouched)
Unknown operationId	Warning (error under --strict)

Constraints

• Requires --operation-id-strategy to be set (not none)
• Overlay file max 10 MiB
• Only numeric response status codes in overlay
• No deep merge of response schemas

Tests

• 8 unit tests in src/enrichments.rs (TDD: RED → GREEN)
• 5 CLI integration tests in tests/enrichments_cli.rs
• All existing tests pass (snapshot_compat, e2e, etc.)
• cargo clippy --all-targets -- -D warnings clean

New files

• src/enrichments.rs — overlay types + apply_enrichments() + unit tests
• tests/enrichments_cli.rs — CLI integration tests