feat(tools): per-scene opcode completeness tracker + 0x259 marker fix

Add tools/scene_opcode_coverage.py: histograms a scene's static opcodes and
classifies each vs the C# VM as impl / safe-noop / GAP (effectful op the VM
silently stubs). Implemented set is parsed live from VirtualMachine.cs case arms
(no drift); metadata from build/opcodes.json. Makes a half-rendered scene legible
("N ops still stubbed") instead of implying everything runs.

SC0000 baseline: 129 distinct ops, ~94.8% instruction-weighted handled, 68 GAP.
The tracker cross-checks opcodes.toml vs VM behavior and surfaced 0x259
(script-entry marker) missing its noop_headless flag -> reconciled in opcodes.toml
and rebuilt (regen: age_opcodes_himegari.py, opcode-reference.md).

Docs: tools-reference.md (tool row), phase-a-slice-plan.md (completeness gauge).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

tools/age_opcodes_himegari.py

@@ -17,4 +17,5 @@ INFERRED: dict[int, dict] = {
     0x1f5: dict(name='stmt-end', category='marker', noop=True, confidence='high', source='investigation', summary='zero-arg; precedes exit/next-stmt, pairs with 0x1f4'),
     0x21b: dict(name='line-id?', category='marker', noop=True, confidence='med', source='harness', summary='1 imm; mov->0x21b->stmt-end; near save/load-messkip — likely line/stmt id, verify not msg-control'),
     0x258: dict(name='decl?', category='marker', noop=True, confidence='low', source='harness', summary='2 imm; runs in a chain right after script-entry 0x259, enumerating ids — prologue declaration/registration?'),
+    0x259: dict(name='script-entry', category='marker', noop=True, confidence='low', source='harness', summary='zero-arg; the first instruction of a script (offset 0), opens the decl chain that 0x258 continues — script/prologue entry marker, structural'),
 }