[YidaWeng]

What

Adds fs.read_bytes(ref, path, offset = 0, length?) to the SDK, CLI and docs, so a file written as bytes can be read back. fs.read remains text only and still refuses a non-UTF-8 file as Binary.

const { bytes, size, hasMore } = await layerfs.fs.read_bytes("feat", "/logo.png", 0, 65536);

Answer: { kind, snapshot, root, path, bytes, size, hasMore }.

• One window at the same immutable snapshot as fs.read (Branch, Commit or Layer).
• length defaults to and is capped at the 48 KiB text window (window::BYTES); page by advancing offset by the returned byte count.
• offset past EOF → empty bytes, hasMore: false.
• IsDirectory for a directory, NotFile for a symlink, NoSuchPath for a missing path, InvalidInput for a negative/unsafe offset or length.
• Engine reads through layerfs_content::file::read_range over the call's read caches (the same byte-window path blame uses); telemetry sdk.bytes. No storage, binding or control-record change — the binding already returns SdkValue::Bytes as a Node Buffer.
• JS surface: @layerfs/agent wrapper, layerfs fs read_bytes <ref> <path> [--offset <n>] [--length <n>], and the agent docs / architecture text.

Note on the issue's decision gate

#417 is marked "Decision needed (owner)". This PR implements the proposed shape from the issue (same method name, same { bytes, size, hasMore } answer, same refusals and 48 KiB cap). If the owner settles on a different shape before merge, this branch is the thing to adjust; nothing is released by opening it.

Tests

• New engine/crates/layerfs-api/tests/sdk_read_bytes.rs: 48 KiB + 1000-byte file read in two windows, cap of an oversized length, offset past EOF, zero length; directory / symlink / missing / negative refusals.
• sdk_method_golden transcript gains six fs.read_bytes lines (whole, offset+length, past EOF, IsDirectory, negative offset, NoSuchPath).
• JS: fixture cases for SDK and CLI, binary_write.mjs round-trip/paging/EOF/refusals, packaging.mjs fs method count 14 → 15.

Checks run (macOS)

• cargo +1.85.1 test -p layerfs-api --test sdk_read_bytes --test sdk_method_golden — 3 passed
• cargo +1.85.1 clippy --locked --all-targets -- -D warnings — clean
• cargo +1.85.1 fmt --all -- --check — clean
• cargo +1.85.1 test --locked --all-targets --no-run — builds
• tools/check_product_boundary.py — PASS (959 files)
• tools unit tests — 155 passed
• node api/agent/test/calls.mjs — PASS (41 cases)
• node --test api/cli/test/cli.test.mjs — 77 passed
• node api/agent/test/packaging.mjs surface|lifecycle — PASS

The focused Rust tests were run with rusqlite bundled enabled locally (host SQLite 3.54.0 is outside the overlay profile); that manifest change is not in the commit.

Not run locally: api/agent/test/binary_write.mjs and the Linux FUSE targets — they need the real native binding and /dev/fuse on Linux.

Production LOC

102763 → 102848 (delta +85): engine +74, api +11, core 0. Counted with tools/production_loc.py --root on git archive of parent 582bc786f and staged tree 9ae38d1f41.