fix(relay): never panic on multibyte request ids when naming worker threads

[duyet]

Summary

The relay named worker threads by byte-slicing the first 8 bytes of a cloud-controlled request id. A multibyte UTF-8 character in those bytes panics (byte index N is not a char boundary). Release is panic = "abort", so one bad frame killed the whole relay.

This truncates the id by Unicode scalar values (chars().take(8)) for the thread name only. The in-flight map still keys on the full id.

Closes #17 (Plan 002).

Checklist

• Version stays on 0.1.x (do not introduce 0.2 / 1.0)
• Do not auto-merge release-please PRs

Skip co-authors

• I skipped co-author trailers on purpose

Plan verification

• Drift check: git diff --stat 61ee3c7..HEAD -- src/relay.rs was empty before the change; live spawn_request matched the issue excerpt.
• Only src/relay.rs changed.
• grep -n 'frame.id\[' src/relay.rs → no matches.
• grep -n 'chars().take' src/relay.rs → helper at the short_id definition.
• cargo build --locked → exit 0 (local sandbox rustc 1.98.0, matching CI @stable).
• cargo test --locked --lib relay → 15 passed, including short_id_is_char_boundary_safe_and_bounded.
• cargo clippy --locked --all-targets → 13 pre-existing lib warnings; none new in relay.rs.

Out of scope (reported, not fixed)

No other untrusted-string (&str[..n]) slices remain in relay.rs. The other .. slices are on raw byte buffers (buf / pending) after utf8_flush_len, plus unit-test fixtures.

  

[github-actions]

anyr size and startup

Startup is wall time for a cold anyr --version / anyr --help (median of 21 runs). Size is the stripped release binary, or the .wasm for the browser demo.

Size budget: anyr-linux-x86_64 must stay ≤ 4.0 MiB (4194304 bytes) stripped.

Asset	Kind	Size	--version median	--help median
anyr-darwin-arm64	native	2.7 MiB	2.88 ms	2.51 ms
anyr-darwin-x86_64	native	2.9 MiB	35.89 ms	33.77 ms
anyr-linux-arm64	native	2.6 MiB	0.7 ms	0.76 ms
anyr-linux-x86_64	native	3.0 MiB	0.95 ms	0.94 ms
anyr-windows-x86_64.exe	native	2.5 MiB	8.72 ms	8.79 ms
anyr.wasm	wasm	175.1 KiB	—	—

Budget check: ok

raw timings

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    "size": "2.5 MiB",
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