Rebuild register allocation around indexed ranges and Fast

[Milky2018]

Summary

• remove post-hoc bridge, residency, edge-affinity, and split heuristics
• replace sorted occupancy arrays with an intrusive range index and merged probes
• eliminate dense edge-transfer scans
• add spillset-preserving gap splits and prioritized second-chance allocation
• replace SinglePass with an independent reverse-scan Fast allocator

Verification

• moon check modules/regalloc --target native --warn-list +73
• moon test modules/regalloc --target native --warn-list +73 (85/85)
• pre-commit native workspace check and 63 script tests
• 12,993-instruction pressure benchmark: Fast median regalloc 22,333 us versus Backtracking 39,275 us; Fast output 78,356 bytes versus 78,264 bytes

The full workspace native test was attempted but the pre-existing MilkIR whitebox executable remained at 100% CPU for over four minutes. ISS-414 records that blocker, so this PR is draft rather than claiming the workspace gate is green.