Per apr-cli-trace-save-tensor-v1.yaml v1.0.0 PROPOSED: integration tests
that exercise the public API of #1133 byte format + #1135 path helpers
exactly as a future apr trace --save-tensor CLI implementation will, and
as apr diff --values will when reading the produced files. These complement
the unit tests in those modules with public-API-surface assertions, catching
regressions that internal tests can miss.

New file crates/aprender-serve/tests/save_tensor_integration.rs (5 tests):

- falsify_apr_trace_save_002_byte_determinism_two_writes
  Two writes with identical inputs MUST produce byte-identical files.
  Partial-discharge of FALSIFY-APR-TRACE-SAVE-002 at the library level.

- falsify_apr_trace_save_004_header_format_via_public_api
  Reads raw file bytes, verifies APRT magic, decodes header via
  parse_header, asserts header.total_file_size() == actual file size,
  decodes f32 LE body element-wise. Partial-discharge of FALSIFY-APR-
  TRACE-SAVE-004 at the public-API surface (complements unit tests).

- falsify_apr_trace_save_005_three_stages_one_layer_independent_files
  Three writes (embedding, ffn_gate, ffn_swigl) at layer 0 produce
  exactly 3 distinct .bin files under layer-0/, each with its own
  correct dim_product. Partial-discharge of FALSIFY-APR-TRACE-SAVE-005
  at the filesystem level (complements parser-level test in #1134).

- whole_model_stages_dont_collide_with_per_layer_zero
  Writes lm_head at WHOLE_MODEL_LAYER and at layer=0; verifies both files
  exist at distinct paths and preserve their own dim_product values.
  Defends against future bugs where the WHOLE_MODEL_LAYER sentinel is
  miscompared at the path-builder layer.

- parse_header_on_truncated_file_errors_via_public_api
  Writes 8 bytes of an APRT header (truncated below the 12-byte minimum);
  parse_header MUST error cleanly. Defends against silent zero-fill on
  filesystem corruption.

Live results from cargo test -p aprender-serve --test save_tensor_integration:
  test result: ok. 5 passed; 0 failed.

Save-tensor contract progress:
- 4 lib modules merged/in-flight (#1133 + #1134 + #1135 + #1136)
- 2 public-API integration tests added (this PR)
- Independent of all in-flight save-tensor PRs (#1134, #1136); compiles
  against the modules already in main from #1133 + #1135.

Five-Whys (Toyota Way):
  Why 1: SHIP-007 next-session priority is per-stage element-wise diff.
  Why 2: Lib-level unit tests cover internal-state invariants well; but
         a public-API caller can violate invariants the unit tests can't
         see (e.g., header offsets at the byte level).
  Why 3: Integration tests against the same public surface that the
         future CLI uses catch a different regression class.
  Why 4: 3 of the 5 falsifiers (002, 004, 005) are partial-dischargeable
         at the integration level today without waiting on the CLI.
  Why 5: §26.8 stack-tool-extension methodology — extend apr in
         falsifier-sized slices.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>