feat: generate C headers from layouts

[tomrford]

Summary

• add mint_core::header::generate and the mint header FILE[#BLOCK]... -o FILE command
• render naturally aligned nested C structs with scalar/fixed-point mappings, reusable array extents, and bitmap shift/mask macros
• validate C identifiers, keywords, field-tree conflicts, ref targets, and generated-name collisions before writing output
• replace the hand-written example header with generated output and update the README, CLI/layout docs, and bundled skill
• cover deterministic rendering, every storage mapping, collisions, full-width bitmaps, and the corrected ABI with a warnings-as-errors C11 compile test

Validation

• nix develop -c cargo fmt --all -- --check
• nix develop -c cargo clippy --workspace --all-targets -- -D warnings
• nix develop -c cargo test --workspace --locked
• nix develop -c uv run --directory crates/mint-python --group dev maturin develop --manifest-path Cargo.toml
• nix develop -c uv run --directory crates/mint-python --group dev pytest tests

Closes #70

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Note

Medium Risk
New public API and CLI surface with substantial header-generation logic; risk is moderate because failures are validated before write and coverage includes C ABI checks, but firmware could still diverge if target alignment differs from Mint’s contract.

Overview
Adds C header generation from the same TOML layout selectors used for flash builds, so the layout can own struct shape for firmware without a data source.

mint_core::header::generate resolves blocks via shared resolve_blocks, builds nested typedef struct trees from dotted paths, maps scalars/fixed-point to <stdint.h> types, and emits array _LEN / _ROWS / _COLS macros plus named bitmap _SHIFT / _MASK macros. It validates C identifiers, keywords, ref targets, field-tree collisions, and upper-snake macro/typedef collisions before returning the full header string; MintError::InHeaderBlock wraps per-block failures.

The CLI gains mint header FILE[#BLOCK]... -o FILE, wired through shared write_text file output. Docs, README, skill text, and doc/examples/blocks.h are updated to the generated format (include guard, extent macros). Tests cover deterministic output, validation errors, unchanged output on failure, and a warnings-as-errors C11 compile of the example header against layout offsets/sizes.

^{Reviewed by Cursor Bugbot for commit f2f79f9. Configure here.}