libc: zero stat fields the kernel does not report

[brandonpayton]

What changed

#907 already landed the mmap and munmap fixes from the old #769 stack and established the current ABI 39 baseline. This PR does not replay that work.

Kandelo's ABI 39 WasmStat ends at 88 bytes. musl's target-specific struct kstat appends st_rdev, st_blksize, and st_blocks, but the kernel does not write those fields. fstatat() previously copied whatever happened to be on the stack into struct stat.

This PR:

• zero-initializes struct kstat before the syscall;
• adds a regression covering fstat(), stat(), and lstat() after poisoning the stack;
• documents the current incomplete metadata honestly.

What was deliberately removed

I did not carry over #769's proposed st_blksize = 4096 or st_blocks = ceil(st_size / 512) synthesis. That calculation is wrong for sparse files because logical size is not allocated storage. I also did not carry over any SQLite patches or harness changes.

#928 tracks the proper follow-up: collect truthful block and device metadata from each filesystem backend and treat the wire-format work as ABI-changing until audited.

ABI and package artifacts

There is no ABI layout or syscall-marshalling change here. ABI_VERSION stays at 39, and the committed snapshot and generated host bindings are unchanged.

There are no package revision, binary-index, or VFS-image changes. Existing static packages remain ABI-compatible; they need an ordinary libc/package rebuild to receive this hardening. No prebuilt package artifacts are published by this PR.

Validation

Run through scripts/dev-shell.sh:

• proved the new focused regression fails on unmodified main by exposing a nonzero garbage st_rdev;
• built musl for wasm32 and wasm64 after the overlay change;
• built the kernel and passed the ABI snapshot/generated-binding check;
• focused stat regression: 1 passed;
• POSIX suite: 174 passed, 3 expected failures, 2 skipped, 0 failed;
• libc suite: 300 passed, 20 expected failures, 1 flaky pass; the 3 initial failures only lacked /bin/sh, and all 3 passed after building current dash;
• Sortix suite: 5,026 passed, 23 expected failures, 53 skipped; the 8 initial failures only lacked generated program/dash fixtures, and all 8 passed after building those fixtures. The new stat regression passed in the full run;
• full program build, including wasm64: passed;
• full host Vitest: 141 files passed, 1,206 tests passed, 2 expected failures, 185 skipped.

Hosted staging CI at head c6f4977f3ea2c6bd909e4daaf3f6fbb01c7ca716 also passed:

• 14 library rebuilds and 57 package rebuilds;
• test-gate preparation;
• cargo/kernel, fork-instrument, POSIX, libc, Sortix, full Vitest, and browser suites;
• merge gate.

Not run locally: manual browser verification, the browser test suite, SQLite sort2, package image rebuilds, or performance benchmarks. The hosted browser suite did run and pass. This PR does not change host runtime, browser adapters, VFS allocation, package recipes, or the ABI, and it makes no performance claim.

[github-actions]

Phase B-1 matrix build status — pr-931-staging

ABI v39. 71 built, 0 failed, 71 total.

Package	Arch	Status	Sha
icu	wasm32	built	597909e9
libcurl	wasm32	built	4c8563da
libcxx	wasm32	built	d4d73697
libcxx	wasm64	built	e5c7a35a
libiconv	wasm32	built	a2e4479a
libpng	wasm32	built	b2c247ba
libxml2	wasm32	built	c7c447be
libzip	wasm32	built	93f64079
openssl	wasm32	built	7eb39510
openssl	wasm64	built	55f7f2e1
sqlite	wasm32	built	532ea231
sqlite	wasm64	built	66859ff4
zlib	wasm32	built	76d2ba71
zlib	wasm64	built	0f0ed605
bc	wasm32	built	54295528
bzip2	wasm32	built	ae4ff490
coreutils	wasm32	built	a744cccd
curl	wasm32	built	7e64cad4
dash	wasm32	built	219bd1fc
diffutils	wasm32	built	85768b7a
dinit	wasm32	built	5645a064
fbdoom	wasm32	built	60fab3c5
file	wasm32	built	64f09e5a
findutils	wasm32	built	304bca9a
gawk	wasm32	built	aabf6e3d
git	wasm32	built	c83f1100
grep	wasm32	built	033701d7
gzip	wasm32	built	228e00c4
hello	wasm32	built	a9c97de2
kandelo-sdk	wasm32	built	4f18529e
kernel	wasm32	built	0f97e289
less	wasm32	built	7f55ef5d
lsof	wasm32	built	424fce40
m4	wasm32	built	4bb05c73
make	wasm32	built	50f15e01
mariadb	wasm32	built	f3d6b0c1
mariadb	wasm64	built	8d6fb52b
modeset	wasm32	built	985e5b34
msmtpd	wasm32	built	6d230c78
nano	wasm32	built	f4895435
ncurses	wasm32	built	e49944d6
netcat	wasm32	built	0b31c929
nginx	wasm32	built	237ced73
php	wasm32	built	95b56839
posix-utils-lite	wasm32	built	d33b0f56
ruby	wasm32	built	3f14ed28
sed	wasm32	built	2b4359f1
spidermonkey	wasm32	built	b9be1e7d
tar	wasm32	built	9606a07c
tcl	wasm32	built	29034860
unzip	wasm32	built	4dd311bd
userspace	wasm32	built	700ab731
vim	wasm32	built	554d5be2
wget	wasm32	built	0c0a53ae
xz	wasm32	built	18fda993
zip	wasm32	built	d8a86605
zstd	wasm32	built	e4b4e772
bash	wasm32	built	870ec43e
mariadb-test	wasm32	built	50dfbbdd
mariadb-vfs	wasm32	built	60b01725
mariadb-vfs	wasm64	built	36e1c9d4
nethack	wasm32	built	765817ec
node	wasm32	built	72d6a54e
spidermonkey-node	wasm32	built	37bcba74
vim-browser-bundle	wasm32	built	5df8c816
nethack-browser-bundle	wasm32	built	a0e0a234
rootfs	wasm32	built	585a598e
shell	wasm32	built	b78ec354
lamp	wasm32	built	49d477ad
node-vfs	wasm32	built	664580bf
wordpress	wasm32	built	08d577dc

_{Auto-generated; replaced on each push. Raw data in the publish-status workflow artifact.}

[brandonpayton]

Closing this focused source PR because its zero-initialization of unreported stat fields is now carried by #934 at exact green head 260d75d5adc31b327f5591229dbfbb287d395fd6. #934 keeps it as a separate purpose commit and passed the full staging, libc, POSIX, and Sortix gates. This is not a claim that this branch was merged verbatim.