Deliver promised public filesystem-stage integration target (T-11.3)

[flyingrobots]

Observable outcome

Deliver the originally promised tests/segment_filesystem_stage.rs public API integration target for completed roadmap T-11.3, without replacing its acceptance requirement with an existing unit-test filename.

Evidence and scope

Original ROADMAP.md at 1a586d8, lines 532–533, names this target alongside tests/segment_writer/. Current main 8d90251 has only tests/segment_filesystem_stage/sandbox.rs and four filesystem laws in src/adapters/filesystem_segment_stage_tests.rs. PR #136 corrected the living v1 ledger but did not create the named integration target. Existing scripted writer tests provide phase-injection coverage; preserve them.

Add meaningful integration laws through the publicly supported filesystem admission/writer/staging boundary. Verify exclusive creation preserves existing bytes, competing stage creation refuses with the exact typed failure, sealing yields canonical golden bytes, and interruption/drop before sealing preserves an explicitly recoverable prefix without producing a sealed receipt. Exercise production admission rather than claiming an unchecked test publisher proves platform admission. Update evidence ownership and the audit after delivery.

Acceptance checks

• The exact named integration target exists and runs via cargo test --test segment_filesystem_stage --all-features in debug and release.
• Laws exercise public API boundaries and exact typed refusals, golden byte evidence, and recovery/prefix semantics; an empty shim or included private unit suite does not satisfy this requirement.
• Existing writer phase-injection, corruption, golden and recovery laws remain green; no evidence is relabeled as a new crash campaign.
• Copy-isolated Docker validation uses an actually admitted filesystem and dedicated source build directory; no forged admission or host tests.
• Required fmt, warnings-denied Clippy, source/documentation checks and CI pass; provide RED-on-parent evidence for the missing target and meaningful regressions for any demonstrated behavior defects.
• Preserve original roadmap checkboxes and link issue, PR and integration commit; T-11.3 is not declared delivered before actual mainline evidence exists.

Exclusions and prerequisites

No byte-format, identity, sealing, publication-order or production durability change. The writable post-seal capability escape belongs to #146; do not duplicate its remediation here. No evidence-backed prerequisite edge to #146 is established merely because both concern stages. If production-path analysis establishes one, record the concrete invariant and dependency before implementation. This is an executable correction under audit #131, not an additional tracking container. Merge must leave main independently working.