perf(string): fuse accumulator concat chains - #8497

Summary

• recognize s = s + a + b + ... for proven string accumulators in local, captured, and module-global slots
• lower the accumulator and suffix operands left-to-right into one rooted runtime call
• reuse a unique accumulator's existing capacity, or allocate the complete result exactly once
• preserve the existing dynamic-add fallback when the string proof is incomplete

Refs #8410.

Profile

The macOS sample utility hung before collecting a sample on this host, including when reproduced against yes. I used the same symbol-preserving build (PERRY_KEEP_SYMBOLS=1 PERRY_DEBUG_SYMBOLS=1, about 17.7k symbols in the generated fixture) with Instruments Time Profiler over a private 10x iso_miss fixture instead.

Baseline sampled CPU was 8,422 ms. The largest leaves were evalNode (38.90%), js_string_concat_chain (14.15%), lookup (12.19%), memmove (4.19%), arena allocation (3.72%), and the separate accumulator append/storage/rooting envelope (6.80% combined).

Candidate sampled CPU was 7,716 ms (-8.38%). The suffix concat plus separate append envelope becomes one js_string_append_chain leaf (15.15%); evalNode remained dominant (42.69%) and lookup remained essentially unchanged (12.08%).

Performance

Five shuffled, interleaved repeats on macOS, comparing a locally built 8d1cca158 baseline with this branch. Both arms used the identical -p perry -p perry-runtime-static -p perry-stdlib-static release build set, and archive mtimes were verified after each build. Medians are shown.

The wall ranges overlap widely (1.41-5.00 s before, 1.24-7.07 s after), so the wall median is not treated as a real 28.9% result. Instructions retired are the primary signal. The observed RSS cost is 48 KiB; the implementation does not add a cache or retained reserve and allocates the complete result once, but the measured peak is reported rather than rounded away.

Whole-corpus medians:

No non-target row moved more than 0.41% in instructions. All non-target wall ranges overlapped; the large wall/cycle swings track host contention rather than instruction count.

Validation

• all 19 corpus programs byte-exact against expected output
• cargo test --release -p perry-runtime --lib: 2,608 passed, 4 ignored
• cargo test --release -p perry-codegen --lib: 1,121 passed
• cargo test --release -p perry --bin perry: 1,008 passed
• lint: initial full run passed 50/52; the two branch-specific ratchets were fixed and pass individually, but the final full 52-gate rerun is blocked by the host disk floor
• gap suite: interrupted after 25/575 when the documented skip-build path reached a nested HTTP-extension Cargo build; a complete retry is blocked by the host disk floor

Host status at handoff: 6 GiB free after deleting this worktree's generated target/ and reducing the reserved external target to about 832 MiB. The task requires stopping below 15 GiB, so this PR remains draft until those two acceptance runs can complete.