feat!: Zig 0.16 migration (std.Io API; BREAKING)

[N283T]

Summary

• Adapt to Zig 0.16's unified I/O interface (std.Io).
• All reader/writer entry points (XtcReader.open, XtcWriter.open, TrrReader.open, TrrWriter.open) gain a new first parameter io: std.Io.
• converter and benchmark migrated to the pub fn main(init: std.process.Init) convention.
• Bump minimum_zig_version 0.15.2 → 0.16.0; package version 0.2.0 → 0.3.0.

Breaking change

Callers must thread io through the API. Obtain it from std.process.Init.io in main, or use std.testing.io in tests.

// Before (0.2.x)
var reader = try XtcReader.open(allocator, "traj.xtc");

// After (0.3.0)
var reader = try XtcReader.open(io, allocator, "traj.xtc");

CHANGELOG.md has the full migration note.

Internal API mapping

0.15.x	0.16
std.heap.GeneralPurposeAllocator	std.heap.DebugAllocator
std.fs.cwd() / std.fs.File	std.Io.Dir.cwd() / std.Io.File
std.io.Reader	std.Io.Reader
std.process.argsAlloc	std.process.Init.minimal.args.toSlice
std.time.Timer	std.Io.Timestamp + Clock.awake
file.getEndPos / file.seekTo	file.length(io) / Reader.seekTo

Test plan

• zig build test — 29/29 pass
• zig build validate — 31/31 pass
• zig build cross-format — 32/32 pass
• converter trr_to_xtc test_data/frame0.trr /tmp/out.xtc succeeds (501 frames)
• CI green on GitHub Actions (Zig 0.16.0)