Handle stacked MultipartRelatedConsolidator with fpp > 1

[thopkins32]

Assisted-by: oh-my-pi:gpt-5.6-sol.

Closes #66

[genematx]

When the default join_method changed from concat to stack, MultipartRelatedConsolidator stopped deriving files_per_datum from chunk_shape and fell back to frame_per_point (default 1). Multi-frame image sequences (e.g. ophyd-async areaDetector TIFF with NumImages > 1) therefore registered a single file per datum, so the declared stacked shape (N, frames, H, W) could not be filled and reads raised a reshape error.

Fix: introduce a single _derive_frames_per_file() helper — the one source of truth for how many frames each file holds (chunk_shape[0] for concat and rank-aligned stacked chunks; 1 for the legacy file-per-frame multiplier/frame_per_point layout; the whole datum otherwise). files_per_datum is then datum_shape[0] // _derive_frames_per_file(), and the physical file layout is recorded in orig_chunks from the same helper. Because both derive from one place they can't disagree, which also fixes sub-array reads (read(0), read((0,1))): previously the legacy multiplier path set files_per_datum but not orig_chunks, so slicing a stacked multi-file datum raised a 422 reshape error. Concat, single- and multi-page rank-aligned stack, and the legacy file-per-frame multiplier cases are all correct for full and sliced reads.

TST: add a parametrized consolidator test (single/2/3-frame, non-divisor, inner-shape-mismatch, legacy multiplier) and a writer test that value-checks full and sub-array reads for single-frame, multi-page, and legacy-multiplier layouts; fix a vacuous assertion in test_tiff_and_jpeg_chunks that never checked the stack file count.

[genematx]

Awaiting Tiled v0.2.19 release before merging.

[codecov]

Codecov Report

❌ Patch coverage is 96.15385% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 78.53%. Comparing base (2d1d76a) to head (76b39a3).

Files with missing lines	Patch %	Lines
src/bluesky_tiled_plugins/writing/consolidators.py	96.15%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #67      +/-   ##
==========================================
+ Coverage   78.38%   78.53%   +0.15%     
==========================================
  Files          16       16              
  Lines        2387     2409      +22     
==========================================
+ Hits         1871     1892      +21     
- Misses        516      517       +1     

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