chore: port to zig 0.16

[laulauland]

Summary

• Ports drift from Zig 0.15.2 to 0.16.0. Pure API migration, no functional changes.
• Threads std.Io through every command and VCS callsite, adopts Juicy Main, and renames to the non-deprecated std.Io.Dir.path / std.mem.find* families.
• Concurrency work using Io.Group / io.async is deferred.

Changes

1. chore: bump to zig 0.16 — mise pin, clap master, min version

• Adds .mise.toml pinning zig = "0.16.0".
• build.zig.zon: minimum_zig_version 0.15.0 → 0.16.0.
• Pins zig-clap to master @ fc1e5cc (first 0.16-capable commit; no tagged release yet).

2. chore: port drift to zig 0.16 — plumbing (juicy main, io threading, writer/path renames) (342+/310−, 12 files)

• pub fn main() → pub fn main(init: std.process.Init); init.gpa replaces GeneralPurposeAllocator.
• CommandContext gains io: std.Io, threaded through every command.
• std.fs.cwd() / openFileAbsolute / accessAbsolute → std.Io.Dir.* (takes io).
• std.fs.File.{stdout,stderr,stdin} → std.Io.File.*; file.close() → file.close(io).
• *std.io.Writer → *std.Io.Writer.
• std.process.Child.run → std.process.run(gpa, io, ...) with stdout_limit = .limited(N) (10 callsites in vcs.zig, lint.zig).
• Child.init + spawn (GitCatFile) → std.process.spawn(io, ...), writeStreamingAll / readerStreaming.
• ArrayList(T) = .{} → .empty; trimLeft/trimRight → trimStart/trimEnd.
• ArrayList(u8).writer(a) → std.Io.Writer.Allocating.
• std.time.milliTimestamp() → std.Io.Timestamp.now(io, .real).toMilliseconds().
• std.fs.path.relative now takes 5 args: (a, "", null, from, to).
• file.readToEndAlloc → file.reader(io, &.{}).interface.allocRemaining(a, .limited(N)).
• Test helpers ported: TempRepo takes std.testing.io; Child.Term tag .Exited → .exited.

3. chore: rename to non-deprecated std.Io.Dir.path and std.mem.find* (60+/60−)

• std.fs.path.* → std.Io.Dir.path.* (aliased, canonical name).
• std.mem.indexOf* → std.mem.find* (aliased, canonical name).

Testing

• zig build passes.
• zig build test passes: 63 tests, 0 failures.
• drift check runs against the real drift repo and produces the expected stale-anchor output.

Known issues

• zig-clap is pinned to master @ fc1e5cc because upstream has not tagged a 0.16 release (latest tag 0.11.0 targets Zig 0.15.1). Bump to a tagged release when one ships.