test(e2e): provide large real-origin input for approved fetch files

[dlddu-k3s-homelab-k3s-mcp]

Problem and result

The real fetch origin returns a 226-byte document, so it cannot supply the >100,000-byte input needed for approval-gated scenario 6(a)'s file result. This adds an opt-in /large-doc[/<session-key>] endpoint serving a deterministic 131,072-byte HTTP response. /doc, its DOCUMENT override, and per-session access-log behavior remain compatible.

Reconciliation task: tbm_session-platform-e2e-mock-policy/rct_20260915-0001.

Plan and scope

1. Receive the successor handoff from tbm_session-platform-scenario-e2e/rct_20260912-0005 in the blocker ledger, including resolution direction, owner, prerequisite and review date (first commit).
2. Resolve it through the existing real origin and document the scenario owner's input contract (second commit). No new mock exception is introduced; the final blocker count is zero because the input cause is resolved.
3. After review, the executor rechecks the exact PR head, current main, CI, fixture responses and policy gates before merging.

The final diff is limited to deploy/fetch-origin.yaml and docs/test/e2e.md. This establishes harness input readiness. Scenario 6's approved fetch, shared-volume assertions and snapshot/restore coverage remain follow-up work; #127, #130 and #142 keep their existing ownership and approval workflows.

Validation

• Extracted the unmodified ConfigMap server.js, checked Node syntax, and served it over real HTTP using Node v22.20.0.
• Default and custom UTF-8 document runs passed: /doc and keyed paths return 226 and 29 bytes respectively; bare and keyed /large-doc return 131,072 bytes with matching Content-Length, exceeding the current maxInlineBodyBytes value of 100,000.
• Large-body SHA-256: 017332cb71752e5bba87a15f650c6c9d882967df97fb35761edb266437fb88df.
• Verified deterministic content across session keys, query-preserving access logs, health/log-read exclusion, and 404 behavior. Both runs recorded exactly their eight served requests. Each local server was terminated after validation.
• Fidelity, scenario mapping, comment policy, render fidelity and git diff --check pass. Seam counts remain 7 registered / 20 accounted / 0 violations; scenario mapping remains 37 = 0 + 3 + 34 + 0.
• Manifest configuration outside the embedded server is byte-identical to main. The deploy/ change triggers the existing kind E2E workflow; its result must be checked for this PR head before merge.

Risk: low; a fixed 128 KiB allocation and an additive path in the kind test origin. Rollback is a reviewed revert of this PR, preserving any later consumer handoff in the blocker ledger.