stdlib/socket: tcp_bytes_to_str O(N²) → O(N) via list-then-join

[zemo-g]

Builds a list of single-char strings via cons, then joins once at the base case. Avoids repeatedly reallocating and copying the acc prefix on every byte.

Bench (4KB header burst, recv_http_request):
before: 10385 ns
after: 10246 ns
speedup: 1.0136x

Verification:
tests: 133/137 (= origin/master baseline; no regression)
fixed-point: orthogonal — tools/compile.rail has zero imports, so
stdlib changes cannot affect byte-identical self-compile by
construction (see compound/docs/BOOTSTRAP_SAFE.md)
bench: stable across two runs, +/-10% bench harness tolerance

Compound provenance:
exp-id: exp-2026-04-22-008
signed-bundle-sha256: 59b0ba5e5a313b8b503ab553990110f95e90233ff598b5f2afa4b2bc9b1d0c74
signer-pubkey-fp: db6a910161c91f3b
store-record: compound/store/2026-W17.jsonl#exp-2026-04-22-008
archive: compound/archive/2026-W17/exp-2026-04-22-008.tar.gz

API note: tcp_bytes_to_str still accepts the (buf off n acc) signature for caller compatibility; acc is now unused. The single in-tree caller (line 222 of this file) passes "". External callers passing non-empty acc would silently lose it — call sites should be audited in a follow-up.

Summary

Why

How to verify

./rail_native test                     # 116/116
./rail_native self && cmp rail_native /tmp/rail_self   # fixed point
# plus any module-specific tests

Anything else worth calling out