fix(context): close evidence and execution policy bypasses

[tcconnally]

Follow-up hardening for the merged #979–#982 context/evidence/runtime work (PR #983), based on an independent exact-tree security review.

This corrective patch closes:

• public-scalar privacy leaks in source references and selection reasons;
• forged or conflicting raw-body evidence digests;
• false-green empty, unavailable, timeout, and not-configured coverage;
• missing top-level abstention propagation;
• execution-profile network escalation, including manually constructed envelopes;
• permissive request/result/capability/profile limits and missing required fields;
• unbounded evidence collections and context-DAG UTF-8 byte budgets;
• malformed and semantically forged self-consistent evidence projections.

Verification completed locally:

• 2479 passed, 6 skipped
• focused security regression matrix passed
• python3 scripts/build.py --check passed
• python3 scripts/render_claims.py --check passed
• generated perseus.py synchronized
• git diff --check passed

The prior implementation remains covered by the already-closed issues; this PR is the post-merge fail-closed hardening follow-up.