Advisories 5.2: delete unused EnforcementMode::Advisory

[gunbai-bot]

Step 5, item 2 of the advisory program (node adhoc-e84e492c-5ef).

Dead by consumer search at origin/main: no lens contract or standing intent constructs Advisory as an EnforcementMode (git grep -nw Advisory over dag/ and src/v2/; the only hits outside prose were the references removed here). The only places it ever appeared were its own declaration, the match arms that consume it, and two test conjuncts.

Consumers removed (§3c):

• v2.lens.enforcement.vocab: the variant itself and its mode_rank arm.
• v2.lens.enforcement.gate: scope_leg Advisory => LegHolds and contract_internally_consistent Advisory => true. Both behaved the same as the AuditOnly arm next to them. The mode_rank ordering below AuditOnly was the only difference, and nothing produced the value, so that ordering could never be reached.
• The import lists in gate and v2.lens.enforcement.contract.
• v2.test.lens_enforcement.gate_test enforcement_mode_order_discriminates: the two conjuncts about Advisory's place in the ordering.

Evidence kept (§4b(4)): enforcement_mode_order_discriminates still has its red and its positive control on the real distinction: AuditOnly does not satisfy Blocking, and Blocking satisfies AuditOnly.

This PR does not touch 00_compile.dag or 04_infer.dag. No gunbc binary is available in this session, so the CI floor is the executing check.

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