IR and codegen backend trait

The codegen-side IR and the CodegenBackend trait (ethercat-esi-codegen (IR + ... (BB_0061)) that lets multiple emitters share it — XML-free, ethercrab-free.

Feature: IR and codegen backend trait FEAT_0052
status: open
satisfies: FEAT_0050
is satisfied by: REQ_0510, REQ_0511, REQ_0512, REQ_0513, REQ_0514
is implemented by: BB_0061

The codegen-side IR (an extension of the parser IR with naming / collision policy applied) and the CodegenBackend trait that lets multiple emitters share that IR. This crate (ethercat-esi-codegen) knows nothing about XML and nothing about ethercrab.
Requirement: CodegenBackend trait shape REQ_0510
status: open
satisfies: FEAT_0052

The crate shall define a CodegenBackend trait with fn emit_device(&self, device: &esi::Device) -> Result<TokenStream, CodegenError> and fn emit_module_root(&self, devices: &[esi::Device]) -> Result<TokenStream, CodegenError>. The top-level entry point shall be fn generate<B: CodegenBackend>(esi: &EsiFile, backend: &B) -> Result<TokenStream, CodegenError>.
Requirement: Naming policy is owned by codegen, not the backend REQ_0511
status: open
satisfies: FEAT_0052
is verified by: TEST_0410

The ethercat-esi-codegen crate shall sanitise ESI product names into valid Rust identifiers (e.g. EL3001-0000 → EL3001_0000) before invoking the backend. Backends shall receive idents pre-validated; they shall not be expected to re-implement sanitisation.
Requirement: Revision collision handled deterministically REQ_0512
status: open
satisfies: FEAT_0052
is verified by: TEST_0411, TEST_0471

When two devices in the input set share a product name but differ in revision (e.g. EL3204 rev 0x00100000 vs rev 0x00110000), the codegen layer shall disambiguate them using a deterministic suffix derived from the full 32-bit revision rendered as eight hex digits (REV{revision:08X}), producing distinct Rust idents (EL3204_REV00100000 vs EL3204_REV00110000). The full-width form is collision-proof (a 4-digit high-word form would alias revisions differing only in the low word). The per-device identity const (Identity const emitted per ... (REQ_0522)) always carries this suffix; the struct name carries it only when a product-name collision requires disambiguation. Ordering of input files shall not affect the generated idents.
Requirement: Common PDO entry types deduplicated REQ_0513
status: open
satisfies: FEAT_0052
is verified by: TEST_0412, TEST_0471

When two or more devices’ PDOs include structurally identical entry layouts (same field order, same bit lengths, same data types), the codegen layer shall emit one shared PDO entry struct referenced by both devices rather than two duplicated structs. Structural equality is the deduplication key — names do not need to match.
Requirement: Emission target is proc_macro2 TokenStream REQ_0514
status: open
satisfies: FEAT_0052
is verified by: TEST_0413

The codegen layer shall produce proc_macro2::TokenStream values and assemble them with quote!. String-templated emission (format! + write) is rejected — token-level construction preserves span / hygiene and yields rustfmt-able output via prettyplease.