chore: code cleanup and documentation overhaul

- Remove debug header dump from MITM proxy (was temp debugging code)
- Suppress dead_code warnings for intentional OpenAI compat fields
- Rewrite README with styled mermaid architecture diagrams, full
  feature listing, usage examples, and CLI reference
- Update endpoint-gap-analysis: images implemented, audio only stretch
- Update mitm-interception-status: add request modification and error
  capture components
- Update standalone-ls-todo: add new endpoints to test results
- Zero compiler warnings

docs/standalone-ls-todo.md

@@ -60,19 +60,25 @@ byte-exact protobuf encoder.
 - API endpoint: `daily-cloudcode-pa.googleapis.com/v1internal:streamGenerateContent?alt=sse`
 - SSE response format: `{"response": {"usageMetadata": {"promptTokenCount", "candidatesTokenCount", "thoughtsTokenCount"}, "modelVersion": "..."}}`
 
-## Test Results (2026-02-14)
+## Test Results (2026-02-15)
 
-| Endpoint                          | Result                    |
-| --------------------------------- | ------------------------- |
-| `GET /health`                     | ✅                        |
-| `GET /v1/models`                  | ✅ 5 models               |
-| `GET /v1/sessions`                | ✅                        |
-| `GET /v1/quota`                   | ✅ real plan/credits      |
-| `GET /v1/usage`                   | ✅ real MITM tokens       |
-| `POST /v1/responses` (sync)       | ✅                        |
-| `POST /v1/responses` (stream)     | ✅ SSE events             |
-| `POST /v1/responses` (multi-turn) | ✅ context preserved      |
-| `POST /v1/chat/completions`       | ✅                        |
-| MITM interception                 | ✅ TLS decrypt + parse    |
-| MITM usage capture                | ✅ per-model token counts |
-| UID isolation                     | ✅ no side effects        |
+| Endpoint                          | Result                      |
+| --------------------------------- | --------------------------- |
+| `GET /health`                     | OK                          |
+| `GET /v1/models`                  | OK, 5 models                |
+| `GET /v1/sessions`                | OK                          |
+| `GET /v1/quota`                   | OK, real plan/credits       |
+| `GET /v1/usage`                   | OK, real MITM tokens        |
+| `POST /v1/responses` (sync)       | OK                          |
+| `POST /v1/responses` (stream)     | OK, full SSE event set      |
+| `POST /v1/responses` (multi-turn) | OK, context preserved       |
+| `POST /v1/responses` (tools)      | OK, function calls captured |
+| `POST /v1/responses` (images)     | OK, MITM injection          |
+| `POST /v1/chat/completions`       | OK                          |
+| `POST /v1/gemini`                 | OK                          |
+| `GET/POST /v1/search`             | OK, grounding + citations   |
+| MITM interception                 | OK, TLS decrypt + parse     |
+| MITM request modification         | OK, tools/images/params     |
+| MITM usage capture                | OK, per-model token counts  |
+| MITM error capture                | OK, instant client feedback |
+| UID isolation                     | OK, no side effects         |