Prepare WSI 0.7 reader and Metal release

[jcwal1516]

This prepares the pending reader and Metal changes for WSI 0.7. Region reads use bounded plans and batches, MIRAX and cache payloads use positional reads, and automatic acceleration calibrates bounded work. It fixes sparse Philips/generic TIFF transparency, oversized synthetic NDPI tile requests, and metadata parsing for supported retired progressive JPEG DICOM syntaxes.

Hamamatsu VMU now opens base/map NGR companions and optional JPEG macros. Native reads preserve RGB16 samples and use bounded positional reads across storage columns. The OpenSlide C shim converts RGB12 tiles before composition, supporting fractional lower-resolution reads and transparent edges. Tests use generated files based on OpenSlide's published NGR layout; real scanner-generated VMU compatibility remains unverified.

Public corpus coverage includes Trestle, Philips-4, generic TIFF, deidentified real SOF2/YBR_RCT/YBR_ICT frames with independent reference pixels, an independently encoded lossless OpenJPH fixture, and a complete source-independent SVCACHE fixture. Separate SlideIO 2.9.0 probes validate native ZVI channel pixels and selected VSI regions, but do not establish full pyramid/geometry parity.

Validation:

• cargo xtask validate passed: 1049 default, 1056 OpenSlide-feature, and 1103 Metal-feature tests, plus formatting, Clippy, release builds and docs.
• All seven VMU integration tests passed with the independent OpenSlide library and freshly built optimized shim, including native samples, malformed input, limits, cancellation, concurrent reads, macro decoding and C ABI region parity.
• Shim tests and warnings-denied Clippy passed; changed-file typos and diff checks passed.
• Earlier focused DICOM tests passed (118), all six public ZVI variants passed, and the NDPI zoom trace now completes 128 reads within the existing limits.

This remains a draft. The unfiltered corpus release gate still requires VMU, Olympus VSI and Zeiss ZVI manifest evidence. J2K dependency updates await J2K 0.11.1 publication. Full RC preflight, dependency/fuzz/performance/platform checks and publication remain outstanding; no release gate has been waived.