Add proof of concept for planar multiband images

[aclark4life]

Refs #10189, #1888.

Changes proposed in this pull request:

• Adds a private, experimental PIL._planar module as a proof of concept for the planar storage design proposed in Add design proposal for multiband and high bit depth images #10189. It may change or be removed without deprecation, and nothing public changes.
• PlanarImage stores each band as an existing single-band core image (L, I;16, I, F), with per-band metadata (name, sample type, role: color/alpha/extra), a colorspace and a premultiplied flag. mode is a single generic value, "planar".
• crop, transpose, resize, rotate and filter are applied per plane. With straight alpha, resize and rotate premultiply color bands first.
• Existing modes (L, LA, La, RGB, RGBA, RGBa, CMYK, I;16, I, F, ...) convert to and from planar images losslessly.

The aim is to test the open questions in #10189 with code before proposing answers.

Findings

1. Per-plane operations work for geometry. For RGB and fully opaque RGBA, crop, transpose, resize, rotate and blur give pixel-identical results to the interleaved image.
2. Single-band modes have gaps. I;16 has no reduce() and no Gaussian, box, median or unsharp filters; F has no BLUR. "No new C code per operation" holds only once these are filled in.
3. F to I;16 conversion clips at 255. Worked around here by converting through I.
4. Premultiplied alpha works at 16 bits. A white edge against transparent black stays white when resized; resizing planes naively darkens it.
5. 8-bit RGBa loses precision at low alpha. Compared with premultiplying in floating point, Pillow's RGBA resize differs by up to 27 levels where alpha is low, and 3–9 where alpha ≥ 128. This supports recording premultiplication as metadata rather than converting around each operation.

Not covered yet

• Opening real high bit depth files (unpacking interleaved data into planes)
• Arrow export of a planar image
• Conversion between sample types
• Integration with Image

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