spec(bazaar): add Observational worked example for Compliance category

[danielnorkin]

Per the conversation on x402-foundation#2285 and x402-foundation#2322, adding the observational worked example to fill the third class in the Compliance category worked-examples section.

The new subsection lives between "Regulatory: Multi-framework compliance oracle" and "Compliance Category: Discovery Filter", matching the shape of the behavioral and regulatory blocks already in this PR.

Structure of the addition:

• Class description + implementor line (Agent 402 Tape)
• YAML registry entry showing the structural fields (signedReceipt: false, anchor_chains == contributing_chains, determinism: aggregating)
• Corresponding PaymentRequired extension fragment (JSON)
• Both structural constraints stated as REQUIRED with rationale
• Outputs reference table for the five aggregations the observational class publishes
• Pointer to the anchor_vs_contributing_constraint_check output as the independent-auditability layer for the behavioral anchor_chains ⊆ contributing_chains rule

No changes to existing content, no schema modifications. Purely additive.

Cross-references:

• Spec discussion: Proposal: Add "Compliance" category to Bazaar discovery + enrich tooloracle-io service x402-foundation/x402#2285
• Privacy axis discussion: Proposal: privacy_class field for Bazaar registry — settlement-plane visibility declaration for compliance-aware x402 services x402-foundation/x402#2326
• Trust-provider hook: feat(extensions): add trust-provider extension for behavioral trust gating x402-foundation/x402#2300
• Sister verascore canonicalization thread: Proposal: Cryptographic Agent Identity extension (CTEF v0.3.1-aligned) a2aproject/A2A#1786