Grid shift file support (NTv2 / NADCON / OSTN15) for sub-meter datum accuracy #248

Context

Issue #243 ships datum support at Helmert accuracy (meter-level residuals for NAD27 and OSGB36). Users with sub-meter/high standards need interpolated grid shifts instead:

• NADCON (.las/.los) — NOAA/NGS, US-government public domain — the accuracy standard for NAD27⇄NAD83/WGS84
• OSTN15 + OSGM15 (Ordnance Survey OpenData, attribution required) — EPSG:7953 (native 1km grid) / EPSG:7709 (NTv2 version) — the OS-recommended route for BNG⇄ETRS89 (cm-level)
• NTv2 generic — mixed bag by file: Canadian NRCan files carry attribution conditions; numerous European/other NTv2 grids are commercial or redistribution-restricted

Design constraints (decided in #243)

• Runtime loading of user-supplied files only — no bundling of grid data without a file-by-file license review (see Support CRSs on other datums (NAD27, OSGB, CH1903+, ...) #243 comment on licensing)
• Files are loaded and a registered CustomProjection/definition wraps them: the pivot contract from Struct-based Projection with an add-only custom registration API (#242) #247 applies, with the grid interpolation replacing Helmert inside the forward/inverse closures
• File formats needed: NTv2 (binary, header + subgrid records), NADCON .las/.los (grid of shifts), optionally horizontal NTv2-ext variants

Rough shape

1. An internal grid type (subgrid lookup, bilinear interpolation) + reader per format
2. A public API to obtain a shift provider from a file path (e.g. GridShift(file:)) used inside CustomProjection closures, or a dedicated definition factory
3. Verification discipline same as Replace Snyder transverse Mercator in UTM with Karney's algorithm #246/Support CRSs on other datums (NAD27, OSGB, CH1903+, ...) #243: independently computed reference values (pyproj wraps PROJ grid shifts) captured as fixtures before porting
4. Docs: per-format notes, license guidance (load-don't-bundle), attribution requirements

References

• Support CRSs on other datums (NAD27, OSGB, CH1903+, ...) #243 (Helmert tier; this is the accuracy follow-up)
• OS: Guide to Coordinate Systems in Great Britain §6.3 (OSTN15, EPSG:7709/7953)
• PROJ ntv2/nadgrids grid format documentation