build: fix zlib compilation on Windows with MSVC (#11798)

## Summary
- Gate `Z_HAVE_UNISTD_H` behind a non-Windows check since `unistd.h`
does not exist with MSVC
- Add `_CRT_SECURE_NO_DEPRECATE` and `_CRT_NONSTDC_NO_DEPRECATE` for
MSVC to suppress deprecation errors for standard C functions that zlib
uses

## Context
Part of the effort to get `zig build -Dapp-runtime=none test` passing on
Windows. This unblocks freetype, harfbuzz, libpng, and dcimgui which all
depend on zlib.

My research shows that we should default to msvc in ci with zig build
ran without `-Dratget`.

## Stack
This is branch 010 in the stacked branches series (soon on Netflix).
Independent fix, no dependencies on other branches.

## Test plan

### test-lib-vt

| | Windows | Linux | Mac |
|---|---|---|---|
| **BEFORE** | 3791/3839 passed, 48 skipped | 3791/3839 passed, 48
skipped | 3807/3839 passed, 32 skipped |
| **AFTER** | 3791/3839 passed, 48 skipped | 3791/3839 passed, 48
skipped | 3807/3839 passed, 32 skipped |
| **Delta** | no change | no change | no change |

### all tests (`zig build test` / `zig build -Dapp-runtime=none test` on
Windows)

| | Windows | Linux | Mac |
|---|---|---|---|
| **BEFORE** | FAIL — 35/51 build steps, 6 failed | 2655/2678 passed, 23
skipped (86/86 steps) | 2655/2662 passed, 7 skipped (160/160 steps) |
| **AFTER** | FAIL — 37/51 build steps, 6 failed | 2655/2678 passed, 23
skipped (86/86 steps) | 2655/2662 passed, 7 skipped (160/160 steps) |
| **Delta** | +2 build steps (zlib + png unblocked) | no change | no
change |

- Zero regressions on any platform
- Windows improved: zlib and png now compile (35 -> 37 steps)
- Remaining 6 Windows build failures (`ssize_t`, `helpgen`, `framegen`,
`harfbuzz`, `dcimgui`) are addressed by other PRs in the stack

## What I Learnt
- Always run tests with `--summary all` to get actual pass/skip/fail
counts. Without it, zig just exits 0 or 1 and you have no numbers to
compare. "You get confident if you got the numbers."
- Build dependencies cascade: fixing zlib also unblocked png (which
depends on it), giving us +2 build steps from a one-file change.

pkg/zlib/build.zig

@@ -32,8 +32,16 @@ pub fn build(b: *std.Build) !void {
             "-DHAVE_SYS_TYPES_H",
             "-DHAVE_STDINT_H",
             "-DHAVE_STDDEF_H",
-            "-DZ_HAVE_UNISTD_H",
         });
+        if (target.result.os.tag != .windows) {
+            try flags.append(b.allocator, "-DZ_HAVE_UNISTD_H");
+        }
+        if (target.result.abi == .msvc) {
+            try flags.appendSlice(b.allocator, &.{
+                "-D_CRT_SECURE_NO_DEPRECATE",
+                "-D_CRT_NONSTDC_NO_DEPRECATE",
+            });
+        }
         lib.addCSourceFiles(.{
             .root = upstream.path(""),
             .files = srcs,