Add bounded exact-byte document extraction protocol and transport

docs/EXTRACTION-ISOLATION.md

@@ -42,7 +42,7 @@ result; retain the original parser-input digest, not a substituted later hash.
 Source files and SQLite are not one atomic transaction. Preserve semantic and
 agent excerpt provenance checks and clear old passages on parser rejection.
 
-- [ ] Buffer API and typed protocol with independent limits and genuine fixtures.
+- [x] Buffer API and typed protocol with independent limits and genuine fixtures.
 - [ ] Headless Rust helper with no GUI dependencies or arbitrary path access.
 - [ ] Parent process deadlines, cancellation, resource setup and kill/reap tests.
 - [ ] Shared-engine document adapter, explicit failures and post-parser guards.
@@ -56,3 +56,24 @@ invalid locations, blocked stdin, worker crash/timeout, missing helper,
 cancelled reconciliation, corrupt/encrypted documents and a source changing
 during parsing. Shipping only protocol types or unit fixtures does not close
 the isolated-extraction release gate.
+
+## Implemented first slice
+
+`searchi-core::extraction_protocol` exposes path-free PDF/DOCX requests and
+validated responses. Version one admits at most 32 MiB input, a 1-KiB JSON
+header, 13 MiB serialized response and 2 MiB aggregate extracted text. Exact
+payload length/SHA, response version/digest and document locations are checked;
+parser rejection is explicit. Unknown nested source fields are rejected.
+
+`extraction_transport` frames bounded JSON lengths as four-byte little-endian
+integers followed by raw input bytes, with no base64 or filesystem paths.
+It rejects oversized lengths before allocation and mismatched input before
+writing. It is blocking I/O: the parent still must supply deadlines, cancellation,
+EOF/process-status validation and owned-child cleanup.
+
+All 50 core tests passed on Windows (18 unit and 32 integration), including
+actual DOCX buffer extraction, corrupt PDF/DOCX rejection, raw binary byte
+preservation, version/hash/location/size failures and truncated frames. Strict
+all-target Clippy, Rust formatting and diff checks passed. Product extraction
+defaults and code/text parsing are unchanged. No separate worker was launched;
+process isolation and resource ceilings remain open.