#1135)

Per apr-cli-trace-save-tensor-v1.yaml v1.0.0 PROPOSED `cli_signature`
invariants: per-layer stages MUST be written under
`<DIR>/layer-<N>/<STAGE>.bin`; whole-model stages (final_norm, lm_head)
go directly under `<DIR>/<STAGE>.bin` with no `layer-N` segment.

This module is the layout primitive that the future apr trace
--save-tensor CLI implementation calls before invoking the byte writer
in inference_trace::save_tensor (PR #1133). Pure-path / minimal-fs slice
keeps the writer and reader (apr diff --values) from drifting apart on
the on-disk layout.

New module crates/aprender-serve/src/inference_trace/save_tensor_paths.rs:
- pub fn output_path(dir, layer, stage_name) -> PathBuf
  * Per-layer: <dir>/layer-<N>/<stage>.bin
  * Whole-model (layer == WHOLE_MODEL_LAYER): <dir>/<stage>.bin
- pub fn ensure_layer_dir(dir, layer) -> io::Result<()>
  * Creates the appropriate parent directory; idempotent

13 unit tests + 1 doctest cover:
- output_path_per_layer_layer_zero, _arbitrary, _absolute_dir
- output_path_whole_model_no_layer_segment (final_norm + lm_head)
- output_path_layer_max_minus_one_is_per_layer (boundary at WHOLE_MODEL_LAYER)
- output_path_nested_relative_dir
- output_path_appends_bin_extension
- output_path_preserves_stage_name_verbatim (no implicit case-fold;
  canonicalisation is the caller's responsibility via SaveTensorStage)
- ensure_layer_dir_creates_per_layer_dir (tempfile)
- ensure_layer_dir_creates_whole_model_dir (no layer-* subdir)
- ensure_layer_dir_is_idempotent
- ensure_layer_dir_creates_nested_parents
- ensure_layer_dir_no_collision_between_per_layer_and_whole_model

Live results: 13 unit + 1 doctest, all green.

Independent of #1134 (stage enum) and depends on #1133 (just merged) for
WHOLE_MODEL_LAYER. Composes with both: the future CLI-wiring PR will
combine SaveTensorStage::canonical_name() + output_path + write_tensor_file.

Five-Whys (Toyota Way):
  Why 1: SHIP-007 next-session priority is per-stage element-wise diff.
  Why 2: Contract `cli_signature` mandates a precise filesystem layout.
  Why 3: Path building is pure formatter logic — fully testable today.
  Why 4: Pinning the layout NOW means writer and reader cannot drift
         (fewer surprises when apr diff --values reads these files).
  Why 5: §26.8 stack-tool-extension methodology — extend apr in
         falsifier-sized slices.

Co-authored-by: Claude Opus 4.7 <noreply@anthropic.com>

…omposition (#1136)

Per apr-cli-trace-save-tensor-v1.yaml v1.0.0 PROPOSED: combine #1133's
byte-format primitives with #1135's directory-layout helpers into a
single ergonomic API. The future apr trace --save-tensor CLI calls
write_stage_file(dir, layer, stage, values) once per (layer, stage)
without managing file handles or paths separately.

New module crates/aprender-serve/src/inference_trace/save_tensor_compose.rs:
- pub fn write_stage_file(dir, layer, stage_name, values) -> Result<PathBuf>
  * ensure_layer_dir → File::create → BufWriter → write_tensor_file → flush
  * Returns resolved path so callers can log it / pass to apr diff
- pub fn read_stage_file(path) -> Result<(TensorHeader, Vec<f32>)>
  * Symmetric one-shot reader for apr diff --values consumers
- thiserror-derived WriteStageError (Io)

10 unit tests cover:
- write_stage_file_per_layer_roundtrip (canonical case)
- write_stage_file_whole_model_roundtrip (no layer-* segment)
- write_stage_file_creates_missing_parent (mkdir -p)
- write_stage_file_truncates_existing (no append behavior)
- write_stage_file_zero_length_tensor (12-byte file)
- write_stage_file_preserves_nan_inf (sign-bit roundtrip via to_bits())
- write_stage_file_header_has_expected_magic_and_layer (raw byte check)
- read_stage_file_propagates_missing_path
- write_stage_file_returns_resolved_path_for_logging (matches output_path)
- write_then_read_three_stages_in_one_layer
  (mirrors FALSIFY-APR-TRACE-SAVE-005 multi-stage scenario at the
   filesystem level: 3 distinct .bin files under same layer-N/)

Live results: 10 passed; 0 failed.

Save-tensor contract progress: 4 modules now in main —
  save_tensor (#1133) + save_tensor_paths (#1135) + save_tensor_compose
  (this PR), plus save_tensor_stage in flight (#1134). The CLI-wiring PR
  now needs only to: parse stages via SaveTensorStage::from_str + call
  write_stage_file at each capture point in the forward pass.

Five-Whys (Toyota Way):
  Why 1: SHIP-007 next-session priority is per-stage element-wise diff.
  Why 2: 3 building blocks already merged (byte format + paths) but no
         single ergonomic API. CLI authors would have to re-derive the
         compose pattern, which invites drift.
  Why 3: A 60-LOC wrapper + 10 tests pins the writer ↔ reader ↔ layout
         invariants in one place, including BufWriter::flush() durability
         + truncating-not-appending semantics.
  Why 4: write_stage_file_returns_resolved_path_for_logging asserts the
         returned path matches output_path() — downstream tooling
         (apr diff --values) relies on this invariant.
  Why 5: §26.8 stack-tool-extension methodology — extend apr in
         falsifier-sized slices.

Co-authored-by: Claude Opus 4.7 <noreply@anthropic.com>

…1137)

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>