re: withdraw the 'unrestorable rbx' claim — the misses are WaitMultiple

The 8 threads whose [rbx] did not resolve to a vtable were never in
XObject::Wait. The backtrace grep matched WaitMultiple as a substring, and
there %rbx is the XObject** array (mov %rsi,%rbx) with the count in %ebp, so
[rbx] is objects[0] -- an object pointer, needing a second deref -- not a
vtable. The unwind restored rbx correctly for all 18.

freeze_waitobj.sh now takes the function and frame index from the backtrace and
applies the matching read, and captures twice in one run (healthy and after the
~270s black-screen) so the comparison is within-run. waitobj_report.py tabulates
both and diffs them, discarding any value info symbol cannot resolve.

tools/re-capture/waitobj_report.py

@@ -0,0 +1,74 @@
+#!/usr/bin/env python3
+"""Turn the gdb wait-object dumps into a table, discarding what does not check out.
+
+The two waiting functions do not put the same thing in %rbx (see
+freeze_waitobj.sh), so they are parsed separately:
+
+    Wait          rbx = this           -> [rbx]        is the vtable
+    WaitMultiple  rbx = XObject**      -> [[rbx+8i]]   is object i's vtable
+                  ebp = count          -> entries past the count are garbage
+
+Every reading is validated the same way: gdb's `info symbol` must resolve it to
+a `vtable for ...` symbol. A polymorphic object's first word always is one, so
+anything else is a misread and is COUNTED but not interpreted. That check is
+what the reading is worth -- without it a stale register looks like a result.
+"""
+import re, sys, collections
+
+VT = re.compile(r'vtable for ([\w:]+) \+ (\d+)')
+HDR = re.compile(r'=== (\w+) T(\d+) (Wait|WaitMultiple) f(\d+) ===')
+
+def parse(tag):
+    """-> list of (thread, kind, count, [vtable-or-None per slot])"""
+    try:
+        txt = open('/tmp/fz-obj-%s.txt' % tag, errors='replace').read()
+    except FileNotFoundError:
+        return []
+    out, cur = [], None
+    for line in txt.splitlines():
+        line = line.replace('(gdb) ', '')
+        h = HDR.search(line)
+        if h:
+            if cur: out.append(cur)
+            cur = dict(th=h.group(2), kind=h.group(3), count=None, slots=[], bad=0)
+            continue
+        if not cur: continue
+        m = re.search(r'rbp\s+0x[0-9a-f]+\s+(\d+)', line)
+        if m: cur['count'] = int(m.group(1)) & 0xffffffff
+        if 'info symbol' in line or line.startswith('$'): continue
+        v = VT.search(line)
+        if v:
+            cur['slots'].append(v.group(1))
+        elif 'No symbol matches' in line:
+            cur['slots'].append(None); cur['bad'] += 1
+    if cur: out.append(cur)
+    return out
+
+def report(tag):
+    recs = parse(tag)
+    print('=== %s: %d wait frames ===' % (tag, len(recs)))
+    if not recs: return collections.Counter()
+    tally = collections.Counter()
+    for r in recs:
+        n = r['count'] if r['kind'] == 'WaitMultiple' and r['count'] else len(r['slots'])
+        n = min(n, len(r['slots']))
+        live = r['slots'][:n]
+        for s in live:
+            tally[s if s else '<unresolved>'] += 1
+        print('  T%-4s %-13s count=%-4s %s' % (
+            r['th'], r['kind'], r['count'] if r['count'] is not None else '-',
+            ', '.join(s or '?' for s in live) or '(nothing readable)'))
+    print('  --- objects waited on:')
+    for k, c in tally.most_common():
+        print('      %-45s %d' % (k, c))
+    return tally
+
+if __name__ == '__main__':
+    tallies = {t: report(t) for t in (sys.argv[1:] or ['healthy'])}
+    if len(tallies) > 1:
+        a, b = list(tallies)
+        print('=== %s -> %s ===' % (a, b))
+        keys = set(tallies[a]) | set(tallies[b])
+        for k in sorted(keys):
+            x, y = tallies[a][k], tallies[b][k]
+            print('   %-45s %3d -> %-3d %s' % (k, x, y, '' if x == y else '  CHANGED'))