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>

vm-map/opcodes.toml

@@ -6644,14 +6644,14 @@ argc = 0
 abi_source = "kelebek+decode-validated"
 
 [opcode.semantics]
-name = "u00416410"
-category = "unknown"
-summary = ""
-noop_headless = false
-source = "kelebek"
+name = "script-entry"
+category = "marker"
+summary = "zero-arg; the first instruction of a script (offset 0), opens the decl chain that 0x258 continues — script/prologue entry marker, structural"
+noop_headless = true
+source = "harness"
 confidence = "low"
 depends_on = []
-evidence = ""
+evidence = "SC0000 offset 0x0 = op 0x259 (argc 0); 0x258's summary names it 'script-entry 0x259'; VM treats it as no-op (default stub) across all 279 CLEAN A0 scenes"
 
 [[opcode]]
 op = 0x2bd