fix(adapters): incremental byte-level SSE parser (FWD-06)

Replace per-chunk parse_sse_frames with SseParser holding raw bytes
across chunks: reassembles frames split at TCP boundaries (the review's
silently-losing case), decodes UTF-8 per complete line so multi-byte
chars split across chunks survive, caps the buffer at 1 MiB
(SSE_EVENT_BUFFER_CAP) and dispatches a pending event at EOF.

forward_stream threads the parser through its unfold state and emits a
terminal error envelope on cap overflow. Existing single-chunk SSE test
assertions preserved; added multi-chunk, split-UTF-8, EOF-dispatch, and
cap tests.

Verified: cargo test (219 pass), clippy -D warnings, fmt --check.

tasks/adapters/review-001-sse-parser.md

@@ -1,7 +1,7 @@
 ---
 id: review-001-sse-parser
 name: Incremental byte-level SSE parser (FWD-06)
-status: pending
+status: completed
 depends_on: []
 scope: narrow
 risk: high
@@ -33,11 +33,11 @@ chunks (decode UTF-8 once over the reassembled buffer, not per chunk).
 
 ## Acceptance Criteria
 
-- [ ] Multi-chunk test: event split across two TCP chunks is delivered (the review's empirically-verified case — the acceptance gate)
-- [ ] Split multi-byte UTF-8 across chunks parses (test)
-- [ ] Pending event dispatched at EOF; trailing partial line length-capped (tests)
-- [ ] Existing single-chunk SSE tests unchanged and green
-- [ ] `cargo test` and `cargo clippy --all-targets -- -D warnings` pass
+- [x] Multi-chunk test: event split across two TCP chunks is delivered (the review's empirically-verified case — the acceptance gate)
+- [x] Split multi-byte UTF-8 across chunks parses (test)
+- [x] Pending event dispatched at EOF; trailing partial line length-capped (tests)
+- [x] Existing single-chunk SSE tests unchanged and green
+- [x] `cargo test` and `cargo clippy --all-targets -- -D warnings` pass
 
 ## References
 
@@ -52,4 +52,49 @@ chunks (decode UTF-8 once over the reassembled buffer, not per chunk).
 
 ## Summary
 
-> Filled on completion.
+> Filled on completion.
+
+**Completed** — FWD-06 fixed via an incremental byte-level SSE parser.
+
+### What changed
+
+- `src/adapters/forward.rs`: replaced the per-chunk `parse_sse_frames`
+  function with `SseParser`, a stateful byte-level parser. It carries
+  the raw undecoded byte buffer across chunks, so a frame split at a
+  TCP boundary reassembles and a multi-byte UTF-8 character split at a
+  chunk boundary is no longer corrupted (UTF-8 is decoded per complete
+  line, after reassembly, not per chunk). Framing kept to what the call
+  protocol needs: lines split on `\n` with optional `\r`, `data:`
+  accumulation joined with `\n` on dispatch, `event:`/`id:`/`retry:`
+  accepted and ignored, comment lines skipped, blank line dispatches,
+  leading BOM stripped, pending event with data dispatched at EOF. A
+  documented `SSE_EVENT_BUFFER_CAP` (1 MiB) caps the reassembly buffer;
+  exceeding it yields `SseParseError::BufferOverflow`, which
+  `forward_stream` converts to a terminal error envelope instead of
+  buffering without bound.
+- `forward_stream` now threads one `SseParser` through the
+  `stream::unfold` state (plus a `broken` flag so a parse/transport
+  error ends the stream) and dispatches the EOF-pending event when the
+  upstream byte stream ends.
+- `src/adapters/from_openapi.rs`: the five existing single-chunk SSE
+  test assertions were preserved (same expected event shapes) and
+  ported to the new `feed` API; added the review's multi-chunk loss
+  case (`"data: {\"n\":1}\n"` + `"\ndata: {\"n\":2}\n\n"` → both
+  events), a split multi-byte UTF-8 test, an EOF-dispatch test, and an
+  oversized-partial-line cap test.
+
+### Verification
+
+- `cargo test` — 219 passed, 0 failed
+- `cargo clippy --all-targets -- -D warnings` — clean
+- `cargo fmt --check` — clean
+
+### Notes for the sequential FWD-07/08/10/12 task
+
+- The parser rejects nothing on malformed UTF-8 inside a well-formed
+  frame (invalid bytes in a `data:` value are ignored — no error); the
+  only error is the buffer cap. If FWD-10/12 adds error-body handling,
+  the `SseParseError` enum in forward.rs is the place to extend.
+- `SseParser::feed` is synchronous and pure; it holds no I/O, so the
+  unfold-state shape `(bytes_stream, parser, broken)` can be reshaped
+  freely without touching parser logic.