build: fix Windows build failures in helpgen and framegen (#11803)

> [!WARNING]
> Review/approve this AFTER #11798, #11800 and #11801 (this PR stacks on
top of rhem... ergo, it includes their commits)
> Don't cheat! Start from the oldest one! 😄 I know these are almost
one-liners but I am doing this mostly for documentation and karma
points. BTW, Github needs to level up this wankflow like a lot... IMHO

## Summary
- Use `writerStreaming()` instead of `writer()` for stdout in helpgen
and main_build_data (`ftruncate` on pipes fails on Windows with
`INVALID_PARAMETER` mapped to `FileTooBig`)
- Replace POSIX `scandir` with `opendir`/`readdir` plus `qsort` in
framegen since `scandir` is not available on Windows

## Context
This fix was previously applied upstream by Mitchell (f4998c6ab) and
reverted 15 minutes later (0fdddd5bc). The reason for the revert is not
clear. Around the same time, a CI step was added to execute cmake
examples on Windows, which was later removed (b723f2a43) with the note
"hangs, so remove it entirely". Whether the revert is related to the
hang or had a separate reason, we don't know.

What we do know:
- Both `helpgen` and `framegen` run during normal builds on Windows (via
`SharedDeps`), not just during dist packaging. Claude had told me the
opposite before but "don't trust and verify".
- Without this fix, both tools fail: helpgen with `FileTooBig`
(ftruncate on pipes), framegen with `scandir` undeclared
- The fix does not regress Linux or macOS

## Stack
Stacked on 012-windows/fix-glslang-msvc.

## Test plan

### Cross-platform results (`zig build test` / `zig build
-Dapp-runtime=none test` on Windows)

| | Windows | Linux | Mac |
|---|---|---|---|
| **BEFORE** (74c6ffe78) | FAIL - 39/51 steps, 4 failed | PASS - 86/86,
2655/2678 tests, 23 skipped | PASS - 160/160, 2655/2662 tests, 7 skipped
|
| **AFTER** (f9d3b1aaf) | FAIL - 44/51 steps, 2 failed | PASS - 86/86,
2655/2678 tests, 23 skipped | PASS - 160/160, 2655/2662 tests, 7 skipped
|

### Windows: what changed (39 > 44 steps, 4 > 2 failures)

**Fixed by this PR:**
- `run exe helpgen` -> was `failure` (FileTooBig from ftruncate on
stdout pipe) -> `success`
- `compile exe framegen` -> was `1 errors` (scandir undeclared) ->.
`success`

**Remaining failures (pre-existing, fixed by later PRs in stack):**
- `translate-c` -> 3 errors (`ssize_t` unknown in ghostty.h on MSVC)
- `compile lib freetype` -> 2 errors (`unistd.h` not found)

### Linux/macOS: no regressions
Identical pass counts and test results before and after.

## Discussion points

### "Grep wider"  other `stdout().writer()` callsites
There are 15+ other `stdout().writer(&buf)` callsites in the codebase.
Build-time generators that capture stdout (webgen, mdgen, unicode
generators) would have the same `ftruncate` issue if they ran on
Windows. Currently they don't appear in the Windows build graph, but
worth noting for future Windows work.

### `writerStreaming()` vs `writer()`
`writer()` calls `ftruncate` on flush/end to set the file size, which
fails on pipes (stdout captured by the build system).
`writerStreaming()` skips the truncate since the output goes to a pipe,
not a seekable file. This is the correct API for this use case on all
platforms, not just Windows.

## What I Learnt
- When upstream has applied and reverted something, state what you
observe rather than speculating about their reasoning. Let the reviewer
fill in context you don't have.
- "Grep wider" (testing pattern): `stdout().writer()` appears in 17
files. Only 2 are fixed here because only 2 are in the current Windows
build path. But the pattern exists more broadly.
- I feel like I am training my replacements. I mean, I am a parent, it
rhymes.
- I feel like my replacements are training me. It rhymes as well.

src/build/framegen/main.c

@@ -8,15 +8,16 @@
 
 #define SEPARATOR '\x01'
 #define CHUNK_SIZE 16384
+#define MAX_FRAMES 1024
+#define PATH_SEP '/'
 
-static int filter_frames(const struct dirent *entry) {
-    const char *name = entry->d_name;
+static int is_frame_file(const char *name) {
     size_t len = strlen(name);
     return len > 4 && strcmp(name + len - 4, ".txt") == 0;
 }
 
-static int compare_frames(const struct dirent **a, const struct dirent **b) {
-    return strcmp((*a)->d_name, (*b)->d_name);
+static int compare_names(const void *a, const void *b) {
+    return strcmp(*(const char **)a, *(const char **)b);
 }
 
 static char *read_file(const char *path, size_t *out_size) {
@@ -54,25 +55,47 @@ int main(int argc, char **argv) {
     const char *frames_dir = argv[1];
     const char *output_file = argv[2];
 
-    struct dirent **namelist;
-    int n = scandir(frames_dir, &namelist, filter_frames, compare_frames);
-    if (n < 0) {
+    // Use opendir/readdir instead of scandir for Windows compatibility
+    DIR *dir = opendir(frames_dir);
+    if (!dir) {
         fprintf(stderr, "Failed to scan directory %s: %s\n", frames_dir, strerror(errno));
         return 1;
     }
 
+    char *names[MAX_FRAMES];
+    int n = 0;
+    struct dirent *entry;
+    while ((entry = readdir(dir)) != NULL) {
+        if (!is_frame_file(entry->d_name)) continue;
+        if (n >= MAX_FRAMES) {
+            fprintf(stderr, "Too many frame files (max %d)\n", MAX_FRAMES);
+            closedir(dir);
+            return 1;
+        }
+        names[n] = strdup(entry->d_name);
+        if (!names[n]) {
+            fprintf(stderr, "Failed to allocate memory\n");
+            closedir(dir);
+            return 1;
+        }
+        n++;
+    }
+    closedir(dir);
+
     if (n == 0) {
         fprintf(stderr, "No frame files found in %s\n", frames_dir);
         return 1;
     }
 
+    qsort(names, n, sizeof(char *), compare_names);
+
     size_t total_size = 0;
     char **frame_contents = calloc(n, sizeof(char*));
     size_t *frame_sizes = calloc(n, sizeof(size_t));
 
     for (int i = 0; i < n; i++) {
         char path[4096];
-        snprintf(path, sizeof(path), "%s/%s", frames_dir, namelist[i]->d_name);
+        snprintf(path, sizeof(path), "%s%c%s", frames_dir, PATH_SEP, names[i]);
         
         frame_contents[i] = read_file(path, &frame_sizes[i]);
         if (!frame_contents[i]) {

src/helpgen.zig

@@ -12,7 +12,7 @@ pub fn main() !void {
     const alloc = gpa.allocator();
 
     var buf: [4096]u8 = undefined;
-    var stdout = std.fs.File.stdout().writer(&buf);
+    var stdout = std.fs.File.stdout().writerStreaming(&buf);
     const writer = &stdout.interface;
     try writer.writeAll(
         \\// THIS FILE IS AUTO GENERATED

src/main_build_data.zig

@@ -34,7 +34,7 @@ pub fn main() !void {
 
     // Our output always goes to stdout.
     var buffer: [1024]u8 = undefined;
-    var stdout_writer = std.fs.File.stdout().writer(&buffer);
+    var stdout_writer = std.fs.File.stdout().writerStreaming(&buffer);
     const writer = &stdout_writer.interface;
     switch (action) {
         .bash => try writer.writeAll(@import("extra/bash.zig").completions),