Support multi-slot broker shared buffers

[Weidong Cui (wdcui)]

This PR adds validated ordered multi-slot shared-buffer sequences across all buffer-bearing broker operations, uses them to support broker-backed file reads and writes up to 512 KiB as one logical operation while preserving existing limits for other operations, increases each association’s shared-buffer pool to 256 slots (16 MiB), and preserves broker-core offset, append, and short-I/O semantics.

[github-actions]

🤖 SemverChecks 🤖 ⚠️ Potential breaking API changes detected ⚠️

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--- failure struct_missing: pub struct removed or renamed ---

Description:
A publicly-visible struct cannot be imported by its prior path. A `pub use` may have been removed, or the struct itself 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/v0.50.0/src/lints/struct_missing.ron

Failed in:
  struct litebox_broker_protocol::shared_buffer::SharedBufferDescriptor, previously in file /home/runner/work/litebox/litebox/target/semver-checks/git-ulitebox/3c316b62778e5fe5f2655e8ee50f174ca751a80a/litebox_broker_protocol/src/shared_buffer.rs:121