refactor(opt,sentinel): split opt.go, add OptimizeLines([]string), and define sentinel error types with errors.Is/As

[0xA672]

Summary

This PR refactors the optimizer package to improve maintainability, performance, and error handling.

Changes

1. OptimizeLines([]string) API — eliminate string round-trips in core.go

• Added OptimizeLines(lines []string, level int) []string and PeepholeOnlyLines(lines []string) []string in vas/opt/opt.go
• core.go switched from strings.Split → strings.Join to direct []string-based pipeline, reducing string allocation overhead during optimization

2. Split 2100-line opt.go into 6 focused files

File	Responsibility
opt.go	Public API (Optimize, OptimizeLines, PeepholeOnly, PeepholeOnlyLines)
fold.go	Constant folding (FoldConstants, foldLine, tokenizeFold)
block.go	Basic-block analysis + dead code elimination (splitBlocks, deadCodeElim, dstReg, readRegs, regIndex, extractLabel, stripComment)
dataflow.go	Data-flow analyses (copy propagation, constant propagation, dead store elimination, strength reduction, store-load forwarding)
peephole.go	Peephole passes (xorZero, testCmp, nopMerge, leaFuse, noopElim, pushPopMov, cancelPairElim, etc.)
aggressive.go	-O2 passes (cse, licm, redundant load elim, pushPopElim, scanPushModPop, tailCallOpt)

3. Sentinel error types (vas/errors.go) — errors.Is/As compatible

var (
    ErrInvalidRegister = errors.New("invalid virtual register (valid: v0-v12)")
    ErrOperandCount    = errors.New("operand count mismatch")
    ErrLintError       = errors.New("lint errors found")
    ErrPreprocessing   = errors.New("preprocessing error")
    ErrUndefinedLabel  = errors.New("undefined label")
)

• 12 operand-count errors in core.go → fmt.Errorf("%w: ...", ErrOperandCount, ...)
• 2 register out-of-range errors in reg.go → fmt.Errorf("%w: %s", ErrInvalidRegister, name)
• 2 undefined-label errors in prep.go → fmt.Errorf("%w: %s", ErrUndefinedLabel, ...)
• 1 recursion-limit error in prep.go → fmt.Errorf("%w: recursion limit exceeded", ErrPreprocessing)
• 1 lint error in core.go → fmt.Errorf("%w (...)", ErrLintError)

Callers can now use errors.Is(err, vas.ErrOperandCount) etc. for precise error classification.

Verification

• go build ./... ✅
• go test ./... ✅ (all packages pass)