Codegen IR and backend trait

The codegen-side IR and the CodegenBackend trait (canopen-eds-codegen (BB_0082)): naming policy, collision handling, and PDO dedup, decoupled from both the INI parser and any transport opinion.

Feature: Codegen IR and backend trait FEAT_0063
status: open
satisfies: FEAT_0060
is satisfied by: REQ_0730, REQ_0731, REQ_0732, REQ_0733, REQ_0734, REQ_0735
is refined by: ADR_0082
is implemented by: BB_0082

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 (canopen-eds-codegen) knows nothing about INI and nothing about taktora-connector-can.
Requirement: CodegenBackend trait shape REQ_0730
status: open
satisfies: FEAT_0063
is verified by: TEST_0623

The crate shall define a CodegenBackend trait with fn emit_device(&self, device: &eds::Device) -> Result<TokenStream, CodegenError> and fn emit_module_root(&self, devices: &[eds::Device]) -> Result<TokenStream, CodegenError>. The top-level entry point shall be fn generate<B: CodegenBackend>(eds: &EdsFile, backend: &B) -> Result<TokenStream, CodegenError>.
Requirement: Naming policy is owned by codegen, not the backend REQ_0731
status: open
satisfies: FEAT_0063
is verified by: TEST_0620

The canopen-eds-codegen crate shall sanitise EDS ProductName strings into valid Rust identifiers (whitespace, hyphens, slashes, dots → _; leading digit prefixed with _) before invoking the backend. Backends shall receive idents pre-validated; they shall not re-implement sanitisation.
Requirement: Revision collision handled deterministically REQ_0732
status: open
satisfies: FEAT_0063
is verified by: TEST_0621, TEST_0681

When two EDS files share ProductName but differ in RevisionNumber (OD index 0x1018:03), the codegen layer shall disambiguate using a deterministic _REV<hex> suffix derived from the raw RevisionNumber. Input file order shall not affect the generated idents.
Requirement: Common PDO entry types deduplicated REQ_0733
status: open
satisfies: FEAT_0063
is verified by: TEST_0622, TEST_0681

When two or more devices’ PDOs include structurally identical entry layouts (same bit-len + data-type tuple list), the codegen layer shall emit one shared PDO entry struct referenced by both devices rather than two duplicated structs. Structural equality is the dedup key; field names do not need to match.
Requirement: Emission target is proc_macro2 TokenStream REQ_0734
status: open
satisfies: FEAT_0063
is verified by: TEST_0623

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 (per Use prettyplease, not rustf... (ADR_0076)).
Requirement: One EDS file equals one device REQ_0735
status: open
satisfies: FEAT_0063
is verified by: TEST_0624

The codegen layer shall treat one EDS file as one device. emit_module_root shall accept &[EdsFile] and emit one device per file plus the shared registry table per Generated module root expos... (REQ_0745).