feat(hpc/blocked_grid): add blocked_grid_struct! macro for SoA-of-grids (PR-X3 B)

The macro generates an SoA-of-grids struct: each named field becomes its
own BlockedGrid<FieldT, BR, BC> with shared rows/cols/padded dimensions.

Generated API (v1 — L1 only; L2/L3/L4 deferred to follow-up):
- {Name}::new(rows, cols) constructor
- rows/cols/padded_rows/padded_cols accessors
- blocks_l1() lockstep iteration → {Name}L1Block<'_>
- map_l1() PRIMARY compute path → new {Name}, input unchanged
- bulk_apply_l1() SECONDARY write-back, carries verbatim
  # Data-flow rule docstring section per .claude/rules/data-flow.md Rule #3
- field_n::<I>() compile-time field accessor (P1 G4 ruling)
- {Name}L1Block / {Name}L1BlockMut / {Name}L1BlockIter view types

Also adds:
- FieldGridRef trait (object-safe dimension accessor for &dyn use)
- Clone impl for BlockedGrid<T: Copy + Default, BR, BC>
- paste = "1" dependency (identifier concat for {Name}L1Block naming)
- pub use paste re-export in lib.rs for $crate::paste::paste! macro hygiene

Reserved field names enforced (compile error if shadowed): `new`, `rows`,
`cols`, `padded_rows`, `padded_cols`, `blocks_l1/2/3/4`, `map_l1/2/3/4`,
`bulk_apply_l1/2/3/4`, `field_n`, `default`.

Inline tests: 2/3/4-field generation, pub/private field visibility,
#[derive(Clone)] passthrough, map_l1 input-unchanged invariant,
bulk_apply_l1 lockstep mutation, field_n::<0>/<1> accessor.

5-gate result: check OK, lib 111/111, doc 79/79, fmt OK, clippy OK.

https://claude.ai/code/session_01UwJuKqP828qyX1VkLgGJFS

Author: claude

Cargo.lock

@@ -98,6 +98,11 @@ fractal = { path = "crates/fractal", default-features = false, optional = true }
 serde = { version = "1.0", optional = true, default-features = false, features = ["alloc"] }
 rawpointer = { version = "0.2" }
 
+# paste — identifier concatenation in macro_rules! expansions.
+# Required by `blocked_grid_struct!` to generate {Name}L1Block / {Name}L1BlockMut
+# view types. Already present in the workspace lockfile (via crates/burn).
+paste = "1"
+
 
 # Cranelift JIT (optional, behind "jit-native" feature)
 # For AVX-512 VPOPCNTDQ/VNNI/VPTERNLOG/BITALG support, use the patched fork: