w6a: branch labels stay out of the symtab (isglobal wired)

Every interned symbol -- including branch labels defined via
p.label -- was emitted STB_GLOBAL, so labels were collide-able
across objects and the header's "GLOBAL symbols only" contract was
enforced nowhere (asym.isglobal was set but never read). Labels
stay interned for fixup resolution; the symtab now carries only
exported definitions, undefined externs, and reloc-referenced syms
(a reloc pre-pass marks those). rt/ensure.o drops from 15 emitted
syms to 2. Bootstrap 991-995 hold byte-identical on the new format.

cmd/w6a/a.h

@@ -50,7 +50,8 @@ struct Asym {
 	int	defined;	/* 1 if we own its address */
 	int	is_text;	/* if 1, address is in .text */
 	int	is_data;	/* if 1, address is in .data (mutually exclusive with is_text) */
-	int	is_global;	/* exported (TEXT) */
+	int	is_global;	/* exported (TEXT/DATA/DATAW) */
+	int	has_reloc;	/* referenced by a relocation — must be in the symtab */
 	u64	addr;		/* offset within section if defined */
 	int	idx;		/* ELF symtab index, filled at emit time */
 	Asym	*next;

cmd/w6a/obj.c

@@ -13,9 +13,9 @@
  * omitted entirely so output stays byte-identical to the pre-DATAW
  * format. Test 991 (selfhost .o byte-diff) depends on this.
  *
- * Symtab indices: 0 = STN_UNDEF, 1 = file (skipped), 2.. = our syms.
- * For simplicity we emit GLOBAL symbols only (no LOCAL ordering rules
- * to worry about).
+ * Symtab indices: 0 = STN_UNDEF, 1.. = our syms (no file entry).
+ * Every emitted symbol is GLOBAL (no LOCAL ordering rules); defined
+ * non-exported label syms are pruned unless a reloc references them.
  */
 #include "a.h"
 #include <stdlib.h>
@@ -156,8 +156,17 @@ a_emit_elf(Asm *a, FILE *f)
 	/* Data symbols carry STT_OBJECT and st_shndx=SH_DATA; everything
 	 * else keeps the legacy STT_FUNC/SH_TEXT shape so non-DATAW
 	 * outputs stay byte-identical. */
+	/* Branch labels are interned for fixup resolution but are NOT
+	 * link-visible: emitting them (all bindings here are GLOBAL)
+	 * made every label collide-able across objects. Only exported
+	 * definitions (is_global), undefined externs, and reloc-
+	 * referenced syms reach the symtab. */
+	for (Areloc *r = a->relocs; r; r = r->next)
+		r->sym->has_reloc = 1;
 	int idx = 1;
 	for (Asym *s = a->syms; s; s = s->next) {
+		if (s->defined && !s->is_global && !s->has_reloc)
+			continue;
 		Sym64 e = {0};
 		e.st_name = stput(&str, s->name);
 		if (s->defined) {

selfhost/cmd/w6a/obj.ww

@@ -187,9 +187,23 @@ export fn emitelf(a: *asm_, fd: i32) i32 = {
 	for (zi < 24) { zsym[zi] = 0u8; zi += 1; };
 	bufputb(&sym, zsym.ptr, 24u64);
 
+	// Branch labels are interned for fixup resolution but are NOT
+	// link-visible: emitting them (all bindings here are GLOBAL)
+	// made every label collide-able across objects. Only exported
+	// definitions (isglobal), undefined externs, and reloc-
+	// referenced syms reach the symtab.
+	let rr: *areloc = a.relocs;
+	for (rr != nil) {
+		rr.asy.hasreloc = 1;
+		rr = rr.rnext;
+	};
 	let idx: i32 = 1;
 	let s: *asym = a.syms;
 	for (s != nil) {
+		if (s.defined != 0 && s.isglobal == 0 && s.hasreloc == 0) {
+			s = s.snext;
+			continue;
+		};
 		let entry: [24]u8;
 		let ei: i32 = 0;
 		for (ei < 24) { entry[ei] = 0u8; ei += 1; };

selfhost/cmd/w6a/opcodes.ww

@@ -170,6 +170,7 @@ type asym = struct {
 	istext:    i32,
 	isdata:    i32,	// mutually exclusive with istext; DATAW symbols
 	isglobal:  i32,
+	hasreloc:  i32,	// referenced by a relocation — must be in the symtab
 	addr:      u64,	// offset within its section (.text or .data)
 	idx:       i32,
 	snext:     *asym,