Allocate broker shared-buffer slots

[wdcui]

Adds operation-scoped shared-buffer descriptors and a FIFO allocator for the 16 local slots. Local callers reserve a slot until response consumption, and the host validates slot use with non-wrapping request IDs. This removes association-wide payload serialization; host execution remains serial until the next PR.

[github-actions]

🤖 SemverChecks 🤖 ⚠️ Potential breaking API changes detected ⚠️

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--- failure method_parameter_count_changed: pub method parameter count changed ---

Description:
A publicly-visible method now takes a different number of parameters, not counting the receiver (self) parameter.
        ref: https://doc.rust-lang.org/cargo/reference/semver.html#fn-change-arity
       impl: https://github.com/obi1kenobi/cargo-semver-checks/tree/v0.49.0/src/lints/method_parameter_count_changed.ron

Failed in:
  litebox_broker_local::BrokerLocal::read_pipe takes 2 parameters in /home/runner/work/litebox/litebox/target/semver-checks/git-ulitebox/4651484ee3ab7783244ff9be170cf13a9d7bc816/litebox_broker_local/src/pipe.rs:47, but now takes 3 parameters in /home/runner/work/litebox/litebox/litebox_broker_local/src/pipe.rs:49
  litebox_broker_local::BrokerLocal::write_pipe takes 2 parameters in /home/runner/work/litebox/litebox/target/semver-checks/git-ulitebox/4651484ee3ab7783244ff9be170cf13a9d7bc816/litebox_broker_local/src/pipe.rs:90, but now takes 3 parameters in /home/runner/work/litebox/litebox/litebox_broker_local/src/pipe.rs:94

--- failure constructible_struct_adds_field: externally-constructible struct adds field ---

Description:
A pub struct constructible with a struct literal has a new pub field. Existing struct literals must be updated to include the new field.
        ref: https://doc.rust-lang.org/reference/expressions/struct-expr.html
       impl: https://github.com/obi1kenobi/cargo-semver-checks/tree/v0.49.0/src/lints/constructible_struct_adds_field.ron

Failed in:
  field WritePipeRequest.buffer in /home/runner/work/litebox/litebox/litebox_broker_protocol/src/pipe.rs:53
  field ReadPipeRequest.buffer in /home/runner/work/litebox/litebox/litebox_broker_protocol/src/pipe.rs:37

--- failure struct_pub_field_missing: pub struct's pub field removed or renamed ---

Description:
A publicly-visible struct has at least one public field that is no longer available under its prior name. It may have been renamed or removed entirely.
        ref: https://doc.rust-lang.org/cargo/reference/semver.html#item-remove
       impl: https://github.com/obi1kenobi/cargo-semver-checks/tree/v0.49.0/src/lints/struct_pub_field_missing.ron

Failed in:
  field length of struct ReadPipeRequest, previously in file /home/runner/work/litebox/litebox/target/semver-checks/git-ulitebox/4651484ee3ab7783244ff9be170cf13a9d7bc816/litebox_broker_protocol/src/pipe.rs:36
  field length of struct WritePipeRequest, previously in file /home/runner/work/litebox/litebox/target/semver-checks/git-ulitebox/4651484ee3ab7783244ff9be170cf13a9d7bc816/litebox_broker_protocol/src/pipe.rs:52

--- failure trait_method_missing: pub trait method removed or renamed ---

Description:
A trait method is no longer callable, and may have been renamed or removed entirely.
        ref: https://doc.rust-lang.org/cargo/reference/semver.html#trait-item-signature
       impl: https://github.com/obi1kenobi/cargo-semver-checks/tree/v0.49.0/src/lints/trait_method_missing.ron

Failed in:
  method with_serialized_payload of trait LocalControlChannel, previously in file /home/runner/work/litebox/litebox/target/semver-checks/git-ulitebox/4651484ee3ab7783244ff9be170cf13a9d7bc816/litebox_broker_protocol/src/channel.rs:69