nx_regalloc_calls.nx

nx_regalloc_calls.nx -- the G1 register allocator's KEYSTONE capability: correct handling of values LIVE ACROSS A CALL. This is the exact bug the bootstrap compiler has (project-knowngood-compiler-regpressure-miscompile): a value held live across a call is left in a CALLER-SAVED register and clobbered by the call, so a later read gets garbage and a comparison flips. The base linear scan (nx_regalloc_linscan) models live intervals but knows nothing about calls or the caller/callee-saved ABI split -- so it can place a live-across-call value in a caller-saved register. That is the gap. This closes it and PROVES it three ways: (1) ra_linscan_calls -- a vreg that spans a call may take only a CALLEE-saved reg, else it SPILLS (a spilled value is in memory and survives any call). (2) ra_validate_calls -- STATIC proof: no live-across-call vreg sits in a caller-saved reg. (3) rc_interp_alloc -- EXECUTION model where a call POISONS caller-saved registers, so a wrong allocation actually computes a wrong answer (the bug, reproduced). The differential (call-unaware diverges / call-aware matches the reference) is the proof. x86-64 SysV ABI: caller-saved = rax,rcx,rdx,rsi,rdi,r8-r11 (clobbered by a call); callee-saved = rbx,rbp,r12-r15 (survive a call). We model regs [0,ncaller) caller-saved, [ncaller,nreg) callee-saved. license_tier: ORIGINAL