w6c+selfhost+lib: cgen quality batch + lib Hare-shape graduation

Six fixes across the toolchain, surfaced by lib/lisp porting work.

  1. f64 compound assigns (`acc += d`, `-=`, `*=`, `/=`). Both stages
     load slot → X1, OP X0 into X1, store back (ADDSD/SUBSD/MULSD/
     DIVSD are reg-reg only). Previous MOVSD-overwrite dropped the
     OP. Locals and top-level lets.

  2. Top-level `[N]u8` arrays + `&arr[i]`. let_emit_size grows a
     TY_ARRAY branch so zero-init DATAW lands; cgindex / N_INDEX
     store / `&base[i]` all detect a global array base and use
     LEAQ name(SB) instead of LEAQ (BP). TK_AMP no longer pre-
     evaluates the operand as a value-load — `&base[i]` computes
     base + i*esz directly. Unblocks Hare's static-buffer pattern:
     strconv.{u64,i64,f64}tos graduate to module-level `*_buf`
     arrays and return owned views.

  3. Cross-module `pkg.Enum.MEMBER`. Nested N_DOT chains that
     don't fold to a known shape now emit `MOVQ <leaf>(SB), AX`
     (mirrors the bare-IDENT unresolved fallback), so isolation
     probes — and the test 990 cgen-match floor — stay consistent
     across stages. strconv exposes `base` as a real `enum i32`;
     callers updated. The `main` exemption (linker entry-point
     keeps bare name even when not exported) mirrors C-side
     collectmods into selfhost cgendecl.

  4. Sum-typed parameter ABI. lib/bytes.{index,rindex} take
     `(u8 | []u8)` needle; lib/strings.byteindex / rbyteindex take
     `(str | rune)` needle (Hare-shaped; the byte-wise misnomer
     `index` is dropped). tagged_arg_size cap bumps to 48 (6 int
     regs), with a new partial-fit branch on the callee: when an
     N-word tagged arg overflows remaining regs, fill what fits and
     stitch the rest from positive BP offsets. scanlocals MCASE
     handles slice binds (24B) and walks each arm with a saved /
     restored seenmark set so two arms naming the same local each
     get their own slot — matches cstage's per-arm scope reset.

  5. 4-reg tagged-return ABI (AX=tag, DX=word0, CX=word1, R8=word2),
     up from 3 regs. Slice-payload variants (`([]T | E)`, slot 32B)
     round-trip ptr/len/cap end-to-end. Every receive site updates:
     let-init via cgwidentaggedstore, match scrutinee spill, cgindex
     tagged-element load (both N_IDENT and fallback bases),
     pushargsrev tagged-ident arg (reads word count from slot size),
     cgreturn slice variant in the shuffle path.

  6. `expr: TaggedAlias` is a widening, not a re-interpret. C cgen +
     selfhost cgwidentaggedstore peel an N_CAST whose destination IS
     the union — so cgexpr's natural shape (str: AX=ptr, BX=len;
     slice: AX=ptr, BX=len, CX=cap) is consumed by the matching
     concrete-variant branch instead of being misread as a tagged
     AX/DX/CX triple. Inner casts to a concrete variant (`7: i32`)
     keep their type for proper tag lookup. `[N]Alias` arrays
     resolve element size via slotsize + aliaslookup, and aliaslookup
     strips a `pkg.` prefix so cross-module references work.

lib/fmt grows `formattable = (i64 | str | bool | rune)` plus
`printv` / `printlnv` taking an explicit `[]formattable` slice (the
receive side of Hare's `args: formattable...`). Call-site variadic
gather isn't wired — callers either hand-build the slice or compose
strconv.i64tos + strings.concat.

700_e2e: 114 → 123 rows (f64 compound, top-level u8 arrays + `&buf[i]`,
pkg.Enum.MEMBER, sum-typed (str|rune) and (u8|[]u8) params, 4-reg
slice-return ABI, formattable array). 26/26 tests, bootstrap stable
through ww4.

selfhost/CLAUDE.md

@@ -27,5 +27,42 @@ Fixed (no workaround needed):
   Wwstage parser + cgen are byte-identical to C cgen on these shapes.
   See cmd/w6c/cgen.c N_RETURN/N_LET/N_DOT/N_MLET, lib/ww/parse/{stmt,
   expr}.ww and selfhost/cmd/wcc/{cgenstmt,cgenexpr,cgenutil,cgendecl}.ww.
+- f64 compound assigns (`acc += d`, also `-= *= /=`) on locals and
+  top-level lets. Both stages now load slot into X1, OP X0 into X1
+  (ADDSD/SUBSD/MULSD/DIVSD register-register), and store X1 back.
+  See cmd/w6c/cgen.c N_ASSIGN float-IDENT branch and
+  selfhost/cmd/wcc/cgenexpr.ww cgassign float local/global.
+- Top-level `[N]T` arrays. The cstage cgen now emits a zero-init
+  DATAW slot and accesses go through `LEAQ name(SB)`; previously
+  the array was filtered out by `let_emit_size` and `arr[i]` fell
+  through to `LEAQ (BP), BX` (off-by-frame). `let_isarray` mirrors
+  the selfhost N_TARRAY path in `letemitsize`.
+- `&arr[i]` (address-of an index). Both stages now compute base +
+  i*esz without a trailing dereference; the previous TK_AMP path
+  pre-evaluated the operand as if it were a value-load. Unblocks
+  Hare's `let s = string { data = &buf, ... }` static-buffer
+  shape. See cmd/w6c/cgen.c N_UN TK_AMP and
+  selfhost/cmd/wcc/cgenexpr.ww cgun TK_AMP.
+- Tagged-union return ABI is now AX=tag, DX=word0, CX=word1,
+  R8=word2 (was AX/DX/CX, 3 words). Slice-payload variants
+  (`(slice | E)`, slot 32B) round-trip end-to-end. Every receive
+  site (let-init, match scrutinee spill, cgwidentaggedstore for
+  call-source, cgindex tagged-element load, pushargsrev tagged-
+  ident arg) reads the fourth word when slot size > 24. See
+  cmd/w6c/cgen.c N_RETURN / cg_widen_tagged_store and the
+  matching selfhost cgenstmt / cgenutil / cgenexpr branches.
+- `expr: TaggedAlias` is a widening, not a re-interpret.
+  cgwidentaggedstore peels an `N_CAST` whose destination IS the
+  union itself, so cgexpr's natural register shape (str: AX=ptr,
+  BX=len) is consumed by the str-payload branch instead of being
+  misread as a tagged AX/DX/CX triple. Inner casts to a concrete
+  variant (`7: i32` in `(i64 | i32)`) keep their type so the
+  scalar branch picks the right variant tag.
+- `[N]Alias` arrays read element size through `slotsize` so an
+  aliased tagged variant (e.g. `[3]fmt.formattable`) takes its
+  full 24B stride per element, not the 8B fallback.
+  selfhost/cmd/wcc/cgenutil.ww slotsize N_TARRAY follows
+  `aliaslookup` on a TNAME element, and `aliaslookup` strips a
+  `pkg.` prefix so cross-module references resolve.
 
 If a port "should work" but the binary is wrong, suspect these first.