chore(ci): pin zig-js at #876, the first revision that builds on Zig 1963

[chrisbbreuer]

main has been red on zig-core since #104 moved craft to 0.17.0-dev.1963. Every failure was inside zig-js at the old pin — that toolchain appends pointer_stability to ArrayList, and zig-js had 27 struct literals plus one JIT layout assertion that named the old shape.

zig-utils/zig-js#876 fixes it upstream — 53/53 checks on the 1818 their CI pins, and 2285 passed / 0 failed on 1963. This is the one-line pin.

Verified locally: craft's full zig build and zig build test with the JS runtime on, against this zig-js content, on 1963 — 153/153 steps, 2519 passed, 0 failed.

Unblocks integration, which has been skipping behind zig-core.

[github-actions]

✅ Binary load time

rounds:    25 interleaved
base:      p50 19.3ms   p95 20.8ms   (18.7–20.9ms)
head:      p50 19.3ms   p95 20.8ms   (18.7–20.9ms)
delta:     -0.2%  (fails above +20.0%)

No binary load time regression.

What this measures

craft --help: process spawn, dynamic linking and argument parsing.
It never opens a window, so it cannot see a change in window or
webview startup — real startup is benchmarks/startup.bench.ts, which
needs a display.

Both binaries are measured interleaved on this runner and compared by
p50, rather than against a number recorded on another machine. On
byte-identical binaries that method reads within ~3.5%; the old one
swung 45%.

[github-actions]

✅ Binary Size Report

Metric	Value
Current Size	14912KB (14.56MB)
Change	0KB (0%) unchanged

Size limits

• Warning: 14.50MB
• Maximum: 16.00MB