Bounded message-type IR

The shared bounded message-type IR (taktora-idl-core (BB_0117)): the plane-generic description of a message — structs, enums, bounded sequences, and request/reply services — onto which every frontend lowers. The message-plane twin of fieldbus-od-core.

Feature: Bounded message-type IR FEAT_0111
status: implemented
satisfies: FEAT_0110
is satisfied by: REQ_0946, REQ_0947, REQ_0948
is refined by: QG_0022
is implemented by: BB_0117

A new crate taktora-idl-core carrying the message-type IR shared by every frontend (DBC today; OMG IDL, ROS 2 later). The IR makes unboundedness unrepresentable by construction so every type has a finite, compile-time-known serialized-length bound.
Requirement: Boundedness is unrepresentable to violate REQ_0946
status: implemented
satisfies: FEAT_0111
is verified by: TEST_0928
links outgoing: BB_0117, TEST_0928

Every type the IR can express shall have a finite, statically computable maximum serialized length. String and Sequence shall each carry a capacity bound in the type itself, so an unbounded string or sequence cannot be constructed. Module shall expose max_serialized_len (and a per-struct variant) returning that bound for any validated module.
Requirement: Structural validation before sizing REQ_0947
status: implemented
satisfies: FEAT_0111
is verified by: TEST_0928
links outgoing: BB_0117, TEST_0928

Module::validate shall reject a module that is not structurally sound: a type that references an unknown struct or enum, a duplicate type or field name, a recursive (directly or transitively self-referential) struct, or a service whose request/response names no declared type. max_serialized_len is defined only for a module that has validated.
Requirement: Plane-generic, policy-free IR REQ_0948
status: implemented
satisfies: FEAT_0111
is verified by: TEST_0928
links outgoing: BB_0117, TEST_0928

taktora-idl-core shall name no wire format, no transport, and no target language. It shall not depend on ethercrab, socketcan, proc-macro2, quote, or any taktora-connector-* crate. Source identifiers shall be carried verbatim; sanitisation to a target-language identifier is a codegen concern (Deterministic naming policy... (REQ_0954)), not an IR one.