fix(reborn): close foundation fail-closed gaps

[serrrfirat]

Summary

Fixes two fail-closed foundation issues surfaced from downstream PR review feedback:

• Caps approval fingerprint canonical JSON traversal depth so deeply nested invocation input returns a controlled HostApiError instead of risking stack overflow.
• Rejects negative USD resource estimates before reservation state mutation so reservations cannot credit active reserved balances.

Changes

• ironclaw_host_api

  • Makes canonical_json(...) return Result<serde_json::Value, HostApiError>.
  • Adds a MAX_CANONICAL_JSON_DEPTH guard.
  • Adds a 10k-deep nested input regression test for invocation fingerprints.

• ironclaw_resources

  • Adds ResourceError::InvalidEstimate.
  • Validates estimate.usd >= 0 in reserve_with_id(...) before acquiring/mutating reservation state.
  • Adds a regression test proving negative USD estimates are rejected and reserved balance remains unchanged.

Verification

cargo test -p ironclaw_host_api -p ironclaw_resources
cargo clippy -p ironclaw_host_api -p ironclaw_resources --all-targets -- -D warnings
cargo fmt --check
git diff --check