refactor(asm)!: collapse linking and cut fused dispatch overhead - #162

Summary

Continues the in-flight asm/prof refactor to a working state, then takes one
pass of quality -> performance -> quality over the runtime.

asm linking collapses onto a single code block. Assembler.Build returns the
finished machine code for one block, so the Code container, its external
relocation table, and the multi-entry Link contract are gone. Buffer owns the
write/execute transition around every install rather than exposing
Unseal/Append/Seal. Register allocation folds into the rewriter that already
rewrote virtual operands, so one linear-scan pass owns the physical bank, the
spill frame, and the label rebasing it forces.

prof keeps one counter table per JIT metric family, keyed by the row's label
set, with rows ordering themselves through a self-referencing constraint instead
of an interface type assertion per comparison.

Fused handlers check stack room once. A fused source stays in a temporary
instead of being pushed, so a fused handler grows the operand stack by exactly
one slot no matter how many sources it folded. Emitting a source's overflow check
only when it pushes on its own removes 5,424 generated lines and every duplicated
check.

Performance

Interleaved A/B, median of five, threaded mode:

Native and adaptive modes are unchanged within noise. No kernel regressed.

docs/benchmarks.md carries a full re-measured cross-runtime table (90 rows).

Rejected simplification

Narrowing dispatch to reload the frame's handler table only on a frame
transition measured 15% slower (724 -> 843 ns on IterativeFib(30)): the
branch misprediction costs more than the unconditional slice reload. Reverted;
the unconditional reload stays.

Testing

• make check (generated-code parity, tidy, fmt, vet, tests, arm64 cross-build)
• go test -race ./...
• coverage 78.8%, above the 72.8% baseline
• cross-builds verified on darwin/linux/windows x amd64/arm64
• two new specs in interp/interp_test.go pin the fused stack-room contract
• internal/jitcheck is a temporary black-box harness proving the JIT still
  lowers, spills, links, and executes through interp's public API after the
  asm collapse

Breaking changes

asm.Link takes one []byte and returns one Callable. asm.Code,
asm.LabelOp, OpPseudoLabel, and RegMask.PopFirst/Count are removed.
Both are within the asm/prof surface where public API changes are in scope.

Summary by CodeRabbit

• New Features

  • Added direct assembly and linking of generated machine-code buffers.
  • Added clear errors for unresolved labels and invalid assembly inputs.
  • Preserved callable entry points when executable buffers grow.

• Improvements

  • Improved ARM64 branch relaxation and register allocation for wide live ranges and recursive calls.
  • Reduced redundant interpreter stack checks while preserving overflow and bounds validation.
  • Improved fused-operation stack handling and JIT execution reliability.

• Documentation

  • Reworked coding, testing, benchmarking, and JIT guidance with updated performance results and clearer requirements.