Serve large-text range reads

Automated verification

Run the focused Core range and service tests:

cd packages/core
npx vitest run src/utils/read-text-range.test.ts src/services/fileSystemService.test.ts

Run the WorkspaceFileSystem, ACP adapter, and HTTP route tests:

cd packages/cli
npx vitest run src/serve/fs/workspace-file-system.test.ts src/serve/bridge-file-system-adapter.test.ts src/serve/routes/workspace-file-read.test.ts

Run the SDK query-serialization tests:

cd packages/sdk-typescript
npx vitest run test/unit/DaemonClient.test.ts test/unit/acpRouteTable.test.ts

Then run repository verification:

npm run lint
npm run typecheck
npm run build

Manual scenario

1. Put a valid UTF-8 CSV larger than 256 KiB in the bound workspace. The original reproduction used a 406,892-byte, 5,000-row CSV.
2. Start qwen serve and connect an ACP session to that workspace.
3. Ask the agent to call read_file with limit: 20 and read the first 20 lines.
4. Confirm the first read_file call succeeds with that finite limit and returns the requested CSV lines.
5. Confirm the agent does not need a shell command such as head, sed, or awk as a fallback.
6. Follow the returned nextCursor until hasMore is false. Confirm the rejoined pages equal the original file and no page returns more than 256 KiB.
7. Append rows after the first page and confirm the outstanding cursor remains valid.

Regression checks

• A no-window read against the same large file remains file_too_large; line-only, maxBytes-only, and line-plus-maxBytes requests are admitted as explicit bounded windows.
• A finite line window over a large binary file remains binary_file.
• A large non-UTF-8 text window is binary_file with a UTF-8 conversion or readBytes hint.
• A supported non-UTF-8 file within the full-snapshot cap still obeys maxBytes after its content is decoded to UTF-8.
• A partial large-file response reports the complete sizeBytes, sets truncated: true, omits the full-file hash, and exposes originalLineCount: null until EOF is known.
• Replacing the pathname, truncating, or overwriting the opened file during the read is rejected; append-only growth remains readable.
• A deep line offset beyond the 8 MiB scan budget returns file_too_large and points the client at cursor paging or readBytes.
• A malformed cursor or a cursor combined with line returns parse_error; a cursor for a replaced or truncated file returns hash_mismatch.

Baseline status

Before the fix, Core could read the requested range from the 406,892-byte CSV, but Serve rejected the file at its 256 KiB full-snapshot gate before slicing. The first bounded-window implementation then required repeated line scans and could not reach pages beyond the scan budget. The focused automated tests cover the corrected Core, WorkspaceFileSystem, ACP, HTTP, and SDK paths; the manual ACP/model scenario remains the release smoke test.