Mettle v0.13.0

Second backend target, first-class closures, and a wave of new optimizer and
tooling work.

BREAKING CHANGE

• The function keyword is now fn. Existing sources using function will not
  compile; rename to fn. All docs and examples have been updated.

AArch64 backend (new target)

• --emit-arm64 compiles scalar Mettle to AArch64 ELF that runs under QEMU:
  from-scratch instruction encoder, AAPCS64 ABI, and executable emit layer.
• Direct ir_aarch64 backend plus a MIR->AArch64 lowering path with vreg
  stack allocation and parameter homing.
• Supported: pointers, arrays, narrow ints, full scalar floats, string
  literals, and write-syscall I/O.

Closures and lambdas

• Anonymous fn(...) lambda expressions.
• Capturing closures with mutations that persist across calls.
• Cross-boundary closures and revived expression-calls.
• Closures and function pointers stored in and called from struct fields.
• A plain function or non-capturing lambda flows anywhere an Fn(...) closure
  type is expected.

Optimizer and IR

• Zero-run PGO (--pgo): interprets main() at compile time to drive
  hot-callee inlining and hotness-aware optimizer thresholds.
• Tail-recursion elimination and layout factorization passes.
• New prefetch and if-conversion IR passes.
• ML-guided IR optimization pass (--ml-opt).
• Per-pass translation validation with automatic quarantine of unsound passes.

Compile-time testing and diagnostics

• @test functions with mettle test (interpreted, assert diagnostics, leak
  detection) and mettle trace.
• mettle explain command, new warnings, richer diagnostics, and multi-error
  recovery.
• Codegen annotation and profiling views in the VS Code extension.

Language

• Local const now accepts float, string, and aggregate values.

Correctness fixes

• Fixed a gather-sum miscompile.
• Fixed float global variables reading back as 0 in the direct-object backend.
• Fixed two aggregate-by-value truncation bugs.
• Const-pool sinking now dominates all uses (#40).

Other

• New benchmark suite and examples; contributor guide; installer and Linux
  build-flag fixes; dead-code cleanup.

What's Changed

• Backend, optimizer, and tooling overhaul: AArch64 target, closures, PGO/TRE, compile-time testing, and ML-guided IR opt by @marquisburg in #44

Full Changelog: v0.12.0...v0.13.0