V3 compiles itself (runtime) — stage-2 fixpoint for a test file

Tag v3-selfcompiles. Main suite 111/111.

The milestone

The self-compiled V3 (/tmp/M2self, built by V3 from its own sources) now runs, parses, and emits code. For a test file its output is byte-identical to the gm2-built V3's:

./M2      tests/t_constfold.mod  ->  gen_ssa/TConstFold.ssa
/tmp/M2self tests/t_constfold.mod -> gen_ssa/TConstFold.ssa
cmp: *** IDENTICAL ***

That is a stage-2 self-hosting fixpoint for t_constfold.

Fixes in this step

Eager OR/AND NIL dereferences — gm2 short-circuits, V3 does not. Rewrote SymTab's guard sites ((x = NIL) OR (x^.…)) into sequential checks, and the driver's PrintListing loop ((nextErr # NIL) AND (nextErr^.line = …)) into a done flag.
Off-by-one in string scans — QbeGen's Len/Cpy/App/ParseInt looped with i < HIGH(s); for a string literal HIGH = len-1, so the last character was dropped. App(fname, "gen_ssa/") produced "gen_ssa", mangling the output file name. Changed to <= HIGH(s).

Status

Self-compiled V3 compiles a small file correctly (byte-identical).
Compiling the whole compiler source set with the self-compiled V3 still aborts (rc 134) — the next runtime bug (likely a capacity or a construct in the compiler's own sources). This is the remaining path to the full 3-stage fixpoint.

Remaining

Fix the self-image abort (diagnose with gdb on /tmp/M2self over the full set).
Cross-stage FileIO selection and bootstrap/fixpoint.sh (Stage1 gm2 → Stage2 V3 → Stage3 V3, byte-identical .ssa), with the stage-aware CR flag.

Files

compiler/src/{SymTab.mod,compiler.frm,QbeGen.mod,M2.lst}.

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