feat: multi-field API — set!(fun, ϕ), op!, and grid/field compatibili…

[ExpandingMan]

…ty checks

Expand the package for contexts that combine several fields at once.

• set! now takes the function first: set!(fun, ϕ). This is breaking (0.1 → 0.2) and makes the do-block form read naturally. Every call site in src, ext, test, docs, examples, benchmarks and the README is updated; the multi-device method in the MDLA extension becomes set!(fun, x, P) for consistency.
• New op!(fun, ϕ, ϕs...; check=true): pointwise in-place update of ϕ from the cell coordinates and the current values of ϕ and any number of other fields, on Fields and forest fields alike.
• Compatibility checks for multi-field functions: grid == (same discretization, device ignored), same_layout, compatible, check_compatible, check_layout, with a first-mismatch property walk, an identity fast path, an out-of-line throw, and regrid-currency checks for forest fields. Variadic forms recurse over the argument tuple and measure 0 B under --check-bounds=yes.
• Unified leaf interface: nleaves, leaf_grid, block, _block_array and _require_current on CartesianGrid/Field, so per-block sweeps can be written once against AbstractGrid/AbstractField.
• Docstrings for set!, op! and grid ==; new names exported; benchmark compat bumped to 0.2.

This is WIP, map! may be causing issues with enzyme.