Fix recursion limit for enums with many variants

[antiguru]

Summary

Fixes compilation failures for enums with many variants (80+) that hit Rust's type recursion limit due to deeply nested Chain<Chain<Chain<...>>> types in derived as_bytes() implementations.

Changes

• Balanced chain tree in derive macro: Replace linear chain(chain(chain(a, b), c), d) nesting (O(N) depth) with a balanced binary tree (O(log N) depth). Supports up to 256 variants.

• #[inline(always)] on Chain::fold: Ensures the balanced chain tree is fully inlined — as_bytes().count() compiles to a single constant for any size enum.

• size_hint and count on Chain/ChainOne: Propagates exact sizes through the tree, making count() O(1) without needing fold. Eliminates an entire iteration pass in encode.

• Eliminate panic paths in encode: Restructure the byte-copying loop to use split_at, chunks_exact, and bytemuck::cast_mut. Removes slice_index_fail and copy_from_slice::len_mismatch_fail from generated assembly.

• Dyn dispatch in encode loop bodies: Route the per-slice closures through &mut dyn FnMut so the inlined iterator tree emits one indirect call per leaf instead of duplicating the closure body at each of the N fold sites. 3× code size reduction with no measurable runtime cost (workload is memcpy-bound).

Assembly verification

Includes examples/enum_as_bytes_asm.rs with a 256-variant enum for cargo rustc --example enum_as_bytes_asm --release -- --emit asm inspection.

Metric (256-variant enum)	Before	After
Compilation	❌ recursion limit	✅ compiles
as_bytes().count()	—	mov w0, #258; ret
encode total code size	—	~12.5K insns
encode panic paths	—	0
small_encode (3 variants)	~300 insns	167 insns

Test plan

• All existing tests pass (cargo test)
• Round-trip example works (cargo run --release)
• 256-variant enum compiles and produces correct assembly
• Benchmarked: no measurable performance regression from dyn dispatch

🤖 Generated with Claude Code