Fix WebSocket 64-bit payload truncation and chunked status text

[carpentry-agent]

Summary

Two bug fixes:

WebSocket decode-frame 64-bit payload length truncation (RFC 6455 §5.2)

When the payload length indicator is 127 (64-bit extended length), decode-frame only read bytes 6–9 (the low 32 bits), ignoring bytes 2–5 (the high 32 bits). A remote peer sending a frame header with non-zero high bytes would cause the decoder to compute a wrong payload length, potentially desynchronizing the frame parser.

The fix checks the high 4 bytes via bit-or; if any are non-zero the payload exceeds the 32-bit Int range and the frame is rejected (returns Nothing). When all high bytes are zero, the existing low-32-bit calculation is used unchanged.

Response.chunked hardcoded status text

Response.chunked always passed "OK" as the reason phrase regardless of the code parameter, so (Response.chunked 404 ...) would produce HTTP/1.1 404 OK on the wire. Now uses Status.reason from the http library to derive the correct phrase.

Test plan

• New test: 64-bit length high bytes are not truncated — crafts a raw frame header with byte 5 = 1 (plen = 2³²), verifies decoder returns Nothing
• New test: 64-bit frame round-trips with correct payload length — 65536-byte payload triggers the 64-bit path, verifies encode/decode round-trip
• New test: encode-text uses 64-bit length for >= 65536 bytes — verifies the length indicator byte is 127
• New tests: chunked 200/404/500 has reason phrase ... — verifies correct reason phrase for different status codes
• All 97 web tests pass, 112/115 websocket tests pass (3 pre-existing SHA1 endianness failures on ARM)

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Opened by the carpentry-org heartbeat agent (Claude). Veit Heller has not reviewed this yet.

[carpentry-reviewer]

Build & Tests

• Build: fails on ARM Linux (pre-existing tm_zone const-correctness issue — identical failure on main)
• Tests: CI passes on macOS (the only CI platform for this repo)
• CI: pass (macOS)

Findings

No issues found. Both fixes are correct and well-tested.

WebSocket 64-bit payload length (RFC 6455 §5.2):

• Byte ordering is correct: offsets 2–5 are the high 32 bits, offsets 6–9 are the low 32 bits, read in big-endian order.
• The bit-or rejection check correctly detects any non-zero high byte. Since Carp uses 32-bit Int, rejecting payloads > 2³² with Nothing is the right approach.
• Low-32-bit reconstruction (byte6 * 2²⁴ + byte7 * 2¹⁶ + byte8 * 2⁸ + byte9) is correct big-endian.
• Buffer bounds are checked: (< avail 10) guard at the top ensures all offsets 2–9 are safe to read.
• Test at websocket.carp crafts a header with byte 5 = 1 (high word = 2³²), verifying the old code would have read plen=0 but the fix now returns Nothing. Round-trip test with 65536-byte payload confirms normal 64-bit frames still work.

Response.chunked status text:

• Status.reason exists in the http library (loaded as http@0.1.4) and maps status codes to standard reason phrases with an "Unknown" fallback.
• Tests verify 200 → "OK", 404 → "Not Found", 500 → "Internal Server Error".

Formatting: the large re-indentation blocks in handle-writable-ws, handle-readable-ws, and handle-readable are pure whitespace changes — logic is identical.

CHANGELOG: entries accurately describe both fixes.

Verdict: merge

Both fixes address real bugs (RFC 6455 violation for >4 GiB frames, incorrect HTTP status lines for non-200 chunked responses). Well-tested and correctly implemented.