Size the browser ID staging for source IDs only

Successor of #1565B, decision 17 (docs/rulings/one-control-abstraction-2026-10-13.md). Code
anchors verified on the batch 8 integration branch codex/controls-batch-8. Issue number:
assigned at filing (written here as #1617). Feature sprint mode (owner, 2026-10-13): one
implementer, normal tests, a batch review.

Product outcome

The browser host reserves ID staging only for the IDs that still cross it. Since #1565B, only
source IDs do: track, submix, route and VCA IDs reach the SDK through the core's control list.

Context

• project_buffers (hosts/host-web/src/lib.rs, the boot path near :1820) sizes the ID staging
  for the longest source, track, submix, route and VCA ID. The test mirrors in
  hosts/host-web/src/tests.rs (near :85 and :827) repeat that sizing.
• #1565B deleted live_control_{track,submix}_{count,id}, the last readers of the non-source IDs.
• Shrinking the staging moves id_staging_bytes in the resource report and the memoryBytes
  rows of hosts/host-web/tests/browser-v1/expected.json.

Decisions frozen for this slice

• D1. The ID staging is sized by the longest source ID alone. Nothing else changes in the
  boot path.
• D2. Re-pin expected.json's memoryBytes rows from a measured browser run, with the before
  and after numbers and the reason in the decision record. The boot-budget multiplier does not
  rise.

Objective gates

1. A host-web test boots a document whose longest track, submix, route and VCA IDs are longer than
   every source ID and shows id_staging_bytes equals the source-ID bound.
2. cargo test --locked -p host-web --features host-web/test-support,
   bash scripts/check-web-audioworklet.sh (boot budget), and the browser qualification for
   chromium (firefox and webkit where installed) with the re-pinned rows.

Test value

• Gate 1 turns red if the staging still reserves room for an ID kind that no longer crosses it.

Authorized paths

• hosts/host-web/src/lib.rs (the staging sizing only), hosts/host-web/src/tests.rs,
  hosts/host-web/tests/browser-v1/expected.json (re-pinned rows only), this spec's decision
  record.

Dependencies

• #1565B. Lane W.

Size

Half a day, one sonnet-high implementer.