6l: port ET_DYN dynamic linking to the ww side

Ports cmd/6l/{dyn,dynout}.c into selfhost/cmd/6l/{dyn,dynout}.ww:
ET_DYN .so loading + PT_INTERP/PT_DYNAMIC ELF emission with .rela.plt,
.gnu.version_r, BIND_NOW. lsym grows dyn fields; pass.ww promotes
undefs to dyn; out.ww dispatches; main.ww takes -L/-l. The ww driver
forwards -L/-l to 6l_ww so 'ww_ww build snake.ww -L /usr/lib -l ncurses
-l c' runs without cc.

Test 996 pins byte-identical output to C-6l on snake.

'make bootstrap' gains a fourth stage with cmp ww3 == ww4, proving
ww3 is byte-stable when used as a compiler — not just a coincidental
two-stage equilibrium.

Four wwstage 6c cgen quirks surfaced and are documented in dynout.ww's
header (two-level field-write through a pointer field, (scalar, str)
tuple returns, def : str, ≤6 arg calling convention).

selfhost/cmd/6l/sym.ww

@@ -6,12 +6,20 @@
 use mem;
 
 type lsym = struct {
-	name:     str,
-	val:      u64,	// offset within combined .text once linked
-	defined:  i32,	// 1 if some lobj defines this symbol
-	owner:    *lobj,
+	name:         str,
+	val:          u64,	// offset within combined .text once linked
+	defined:      i32,	// 1 if some lobj defines this symbol
+	owner:        *lobj,
 	idx_in_owner: i32,
-	snext:    *lsym,
+	// Dynamic-linking fields. Set by l_resolve when an undefined sym
+	// is provided by some loaded lso. plt_idx and dynsym_idx default
+	// to -1 (set explicitly by l_resolve, not by amalloc-zeroing).
+	is_dyn:       i32,
+	dyn_lib:      *lso,
+	dyn_version:  str,	// matched export's version; len 0 if none
+	plt_idx:      i32,
+	dynsym_idx:   i32,
+	snext:        *lsym,
 };
 
 type lrel = struct {
@@ -31,15 +39,33 @@ type lobj = struct {
 	onext:     *lobj,
 };
 
+// lexport — one entry per GLOBAL/WEAK symbol exported by a loaded .so.
+// Stored as a chain in the order the .so's dynsym presents them, so
+// l_so_provides_v's first-match semantics agree with the C version.
+type lexport = struct {
+	name:    str,
+	version: str,	// len 0 for unversioned globals
+	enext:   *lexport,
+};
+
+type lso = struct {
+	path:    str,	// full filesystem path used to load
+	soname:  str,	// DT_SONAME, or basename if missing
+	exports: *lexport,	// dynsym-order chain of exported names
+	sonext:  *lso,
+};
+
 type lnk = struct {
 	a:       *arena,
 	objs:    *lobj,
+	sos:     *lso,
 	syms:    *lsym,
 	rels:    *lrel,
 	text:    *u8,	// combined .text
 	textcap: u64,
 	textlen: u64,
 	errs:    i32,
+	dyn_n:   i32,	// number of syms routed through PLT
 };
 
 fn streq(a: str, b: str) bool = {
@@ -58,7 +84,7 @@ export fn l_intern(l: *lnk, name: str) *lsym = {
 		if (streq(s.name, name)) { return s; };
 		s = s.snext;
 	};
-	let n: *lsym = amalloc(l.a, 64u64): *lsym;
+	let n: *lsym = amalloc(l.a, 96u64): *lsym;
 	n.name = name;
 	n.snext = l.syms;
 	l.syms = n;