conductor(campaign): metadata_ssdl_defusing_20260624 - 3-child SSDL defusing campaign

Campaign: address the parent code_path_audit_20260607 Finding 1 (CRITICAL)
Metadata 4.01e22 effective codepaths via 3 SSDL techniques.

3 children, sequential, with budget gates:
1. metadata_nil_sentinel_20260624 (>= 10% drop): introduce
   NIL_METADATA sentinel + migrate 6 nil-check functions.
2. metadata_generational_handle_20260624 (>= 20% drop,
   BLOCKED_BY 1): wrap Metadata in (index, generation) handle;
   collapse lifetime branches to 1 lookup + 1 cmp.
3. metadata_field_cache_20260624 (>= 30% drop, BLOCKED_BY 2):
   MetadataFieldCache keyed by (handle.index, field_name);
   123 string-keyed entry.get('key', default) sites become
   cache lookups.

Each child has its own spec/plan/metadata/state. Budget gate
after each child: re-measure effective codepaths; if drop < threshold,
PAUSE the campaign and report to user.

End-of-campaign TRACK_COMPLETION captures the cumulative reduction
vs the 4.01e22 baseline. Deferred follow-up: apply the same
3 SSDL primitives to the 4 other dict[str, Any] aliases
(FileItem, CommsLogEntry, HistoryMessage, ToolDefinition, ToolCall).

16 files committed: 4 directories x 4 files each (spec, plan,
metadata, state).

conductor/tracks/metadata_nil_sentinel_20260624/state.toml

@@ -0,0 +1,43 @@
+# Track state for metadata_nil_sentinel_20260624
+# Child 1 of metadata_ssdl_defusing_20260624 campaign.
+# 3 phases, 3 tasks. Tier 2 to execute per conductor/workflow.md.
+
+[meta]
+track_id = "metadata_nil_sentinel_20260624"
+name = "Child 1: Metadata Nil Sentinel"
+status = "active"
+current_phase = 0
+last_updated = "2026-06-24"
+
+[parent]
+parent_campaign = "metadata_ssdl_defusing_20260624"
+
+[blocked_by]
+code_path_audit_20260607 = "shipped"
+
+[blocks]
+# Within the campaign:
+metadata_generational_handle_20260624 = "pending child 1"
+
+[phases]
+phase_1 = { status = "pending", checkpointsha = "", name = "Behavioral Test" }
+phase_2 = { status = "pending", checkpointsha = "", name = "Implementation (NIL_METADATA + 6 migrations)" }
+phase_3 = { status = "pending", checkpointsha = "", name = "Verification + Budget Gate" }
+
+[tasks]
+t1_1 = { status = "pending", commit_sha = "", description = "Write tests/test_metadata_nil_sentinel.py with 2 tests (red)" }
+t2_1 = { status = "pending", commit_sha = "", description = "Add NIL_METADATA constant + migrate 6 nil-check functions" }
+t3_1 = { status = "pending", commit_sha = "", description = "Run all 6 VCs; capture budget gate measurement; write TRACK_COMPLETION; update state + tracks.md" }
+
+[verification]
+vc1_nil_metadata_defined = false
+vc2_6_nil_checks_migrated = false
+vc3_behavioral_test_passes = false
+vc4_budget_gate_met = false
+vc5_full_test_suite_green = false
+vc6_audit_gates_clean = false
+
+[budget_gate]
+baseline = 4.01e+22
+expected_drop_pct = 10
+post_child_1_measurement = null