The driver flip moves build artifacts from next-to-source <stem>.s to a
.sepwork/ scratch dir. 29 Pattern-B gates now build `ww build --sep -o <stem>`
and read <stem>.sepwork/__root.s (multi-package gates concat all
<stem>.sepwork/*.s, since cross-package labels live in per-package .s).
All intermediates redirect to /tmp (WW_PKGCACHE + -o), so the corpus runs
parallel-safe with no source-tree pollution. Tests pass now (--sep is live)
and survive the flip.
915 additionally retargeted off strconv's PRIVATE left_shift_table (a let,
not export) — which separate compilation correctly hides — onto a test-local
package that exports its own probe table (#96). The combined path only linked
it via a single-unit private leak; encapsulation is now honored under sep.
Test-only; all 5 *_ww binaries HOLD. 989_m1mangle_run deferred (blocked by
#99, imported-package fn main mangling under sep).
Variant selection (cg_tag_for_variant / flatvariantidxt) matched union variants by exact type only, so widening a bare value (e.g. *vtable) into a union with a NAMED ptr-alias variant (stream = *vtable, in handle = (file | stream)) found no match and the tag defaulted to 0 -- the wrong variant. In the compiler this hit emitbytes' io.write(&cgoutstream.vt) once io.write took a handle, writing the asm to a garbage fd -> empty .s -> w6c_ww miscompiled everything. Add a second selection pass: when the exact pass finds no variant, structurally compare the bare source against each NAMED-alias variant's unwrapped type; exact-match still wins in pass 1 (so a bare i64 stays the i64 variant, not oserror=!i64, which kept the os/errno union building). A >=2-structural-match collision guard (extending #218's) hard-errors LOUDLY on genuine nominal ambiguity (two ptr-aliases to the same struct) instead of silently first-picking, citing #199b/#10. Symmetric across cstage (cmd/w6c/cgen.c) and wwstage (selfhost/cmd/wcc/cgenutil.ww). One-level NAMED unwrap (chained ptr-aliases unmatched, unexercised -> #17). Adds test/wcc/789 (positive widen byte-id+runtime + degenerate-ambiguity reject guard, both stages). Unblocks post-eFinal #5's handle surface. rule-10 fix-up.
