feat(core): add grant intent obstruction receipt shape

[flyingrobots]

Summary

Adds the next Echo authority-ladder vertebra: obstruction receipts for capability grant intent refusal.

Core doctrine:

Refusal is a first-class causal event.

This PR makes every obstructed grant intent carry a CapabilityGrantIntentObstructionReceipt containing:

• intent id
• proposed-by principal
• subject principal
• obstruction reason
• policy id, if present
• policy posture
• deterministic length-prefixed receipt input bytes
• BLAKE3 receipt digest

Naming guard

ObstructionReceipt != AdmissionReceipt
ObstructionReceipt != LawWitness
ObstructionReceipt != accepted authority

What this proves

• obstructed grant intents return posture plus receipt-shaped refusal
• receipt input bytes are deterministic
• receipt digest is deterministic BLAKE3 over the input bytes
• repeated identical refusal inputs produce identical outcomes
• no grant becomes authority

What this is not

• not a success receipt
• not an admitted grant
• not authority validation
• not invocation success
• not LawWitness emission
• not scheduler, WASM, app, or Continuum work

Verification

RED:

cargo test -p warp-core capability_grant_intent
# failed before GREEN because CapabilityGrantIntentObstructionReceipt and posture.receipt did not exist

GREEN / validation:

cargo test -p warp-core capability_grant_intent
cargo check -p warp-core
scripts/ban-nondeterminism.sh
git diff --check
npx markdownlint-cli2 docs/design/optic-capability-grant-intent-boundary.md CHANGELOG.md

Push gate also passed: fmt, guards, clippy-core, tests-warp-core, rustdoc.

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📥 Commits

Reviewing files that changed from the base of the PR and between a27ac47 and f1d5272.

📒 Files selected for processing (5)

• CHANGELOG.md
• crates/warp-core/src/lib.rs
• crates/warp-core/src/optic_artifact.rs
• crates/warp-core/tests/capability_grant_intent_tests.rs
• docs/design/optic-capability-grant-intent-boundary.md

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch stack/grant-intent-obstruction-receipt

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[flyingrobots]

Superseded by PR #339. The useful pieces from this branch were reconciled into #339: policy id/posture context on the generic ObstructionReceipt plus explicit refusal-context and determinism coverage. Keeping #339 as the single path avoids preserving the older specialized CapabilityGrantIntentObstructionReceipt shape.