chore: restore green CI on the current toolchain

[bobzhang]

main currently fails two of its own CI gates on the released toolchain (moon 0.1.20260827 / moonc v0.10.11+6ff76a5f9). The last green run was 2026-08-22, so the toolchain appears to have moved since. Any PR opened today goes red before its tests ever run — that is how I noticed, in #32.

Two commits, each a separate concern:

1. fix: replace deprecated StringBuilder::new() with StringBuilder()

moon check --deny-warn (CI step 6) fails:

internal/header/header.mbt:41:13
  let buf = StringBuilder::new()
  Error Warning (deprecated): Use `StringBuilder()` instead

Six occurrences across index.mbt, internal/header/header.mbt, utils.mbt.

This is the one that actually blocks everything: moon check failing means steps 7–10 are skipped, so moon info, moon fmt and moon test never run at all on any PR.

2. chore: apply moon fmt

moon fmt + git diff --exit-code (CI step 8) fails. 15 files, formatting only. moon fmt is at a fixed point afterwards — re-running produces nothing further.

Verification

All four CI steps, run locally on this branch with the toolchain CI installs:

step	result
moon check --deny-warn	exit 0
moon info --target native + git diff --exit-code	exit 0
moon fmt + git diff --exit-code	exit 0
moon test --target js,native	exit 0 — 105 passed (js), 100 passed (native)

Relationship to #32

#32 (replacing the removed moonbitlang/core/strconv) is currently red for exactly these reasons, not for anything it changes. I kept the two apart so a 7-line dependency fix would not arrive buried in a 17-file reformat.

With this branch as the base, #32's change passes all four gates — I verified the combination locally (--deny-warn 0, fmt --check 0, 105/100 tests). So merging this first should turn #32 green with no changes needed there.

Entirely happy for you to take only the first commit, or to close this if the formatting is intentional — the StringBuilder one is the blocker; the moon fmt one is just the next gate behind it.

🤖 Generated with Claude Code

https://claude.ai/code/session_01AmoyPMELemz5hdNNdXyAkm