test(anchor): fix proof-only receipt fuzz oracle for #533

[dinakarjs]

A valid proof-only receipt now passes TR-ANC-002 using the supplied registry entry, but the fuzz target's post-PASS assertion reparses it as a self-contained receipt and raises InclusionError. This reproduces the missing-leaf_count failure in ClusterFuzzLite run 38987830178 (job 117263304673).

This companion targets #533's branch at 6a9d803. The post-PASS oracle parses index/path from the proof and root/count from the entry, then independently replays inclusion. It retains the standalone parse_receipt exception-safety probe and the expected-claim/root assertions.

Five deterministic tests exercise the actual TestOneInput callback with only Atheris instrumentation and raw-byte selection stubbed: a self-contained receipt; each optional copy omitted individually; both omitted; and a deliberately false module PASS over an invalid proof, which the independent replay must catch. The parser, verifier and callback assertions are real.

Validation on Python 3.12: full conformance suite 823 passed, 5 xpassed; Ruff on both changed files and diff whitespace check pass. A process-memory restoration of the old oracle produces four expected failures and one passing complete-receipt control; source files remain unchanged. Local validation is deterministic harness coverage. Hosted exact-head ClusterFuzzLite run 39007093181 subsequently passed both its build and Run fuzzers steps; CI, Docs, Conformance docs and package build also pass. No claim is made about an extended fuzz campaign. Independent review remains required before incorporation.

No normative or runtime verifier changes. The 30-day breaking-change comment period and Project Lead sign-off for #533 remain required. URI-to-entry linkage and entry-origin authentication remain outside this fix.

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[dinakarjs]

Publication check: head 81b3e8d preserves the fuzz target's existing executable mode. Its complete tree 2365dcccc2e1ea83c0c086e9cb14a72e275f9dc5 exactly matches the locally validated tree (823 passed, 5 xpassed); no content changes since that validation. Hosted fuzz results remain pending.