docs(plans): book step 15's landed files in the S30 plan

Summary

Corrects Step 15's entry in the S30 plan against the tree the step lands on, and books the files the implementation actually touches.

Three bookings, all surfaced by the step's validation pass:

• internal/format/maven/handler.go — the composition the step's own Scope names ("composed inside maven.NewHandler") lives here, not in the wire: the five Deps virtual seams, composeMavenVirtualResolution, and the VirtualResolution() accessor. No earlier step books these lines; none of them existed at Step 11b's tip.
• cmd/artifact-registry/remote_audit_sink_test.go — the production sink's own tests: the payload table over every UpstreamExclusionReason column value and the Info-floor subtest, beside the wiring suite's concrete-type pin the entry already books.
• docs/dev/observability.md — the exclusion audit record's catalog row, which the observability guardrail requires in the same MR as the record and which no step in this plan books (19a/19b book only the metrics and wide-event rows).

Merge order

This MR is the approval alignment for Step 15's own MR, !2947 (merged), which names this MR as its merge-order prerequisite: it lands first, so the plan on main books the eight paths the step MR touches rather than four of them.

No step MR edits the plan file or its Status table: Step 15's row is recorded by the standing table MR !2693 (merged), which owns that table.

Tests

Docs-only: no code changes, and no docs/testing/ scenario is added or affected.

Size

16 insertions in one file (docs/plans/2026-09-10-s30-maven-virtual.md), under the 500-line reviewable ceiling.