Reduce ulitebox broker startup latency

[Weidong Cui (wdcui)]

This PR reduces the startup latency of Linux-on-Linux ulitebox runs with the out-of-process broker (median wall time, base → this PR: static hello 9.1 → 6.6 ms, dynamic hello 21.6 → 13.5 ms, bash -c true 27.4 → 17.0 ms, python3 -c pass 71.0 → 45.7 ms, node -e 0 186 → 84 ms). The broker's overlay now resolves paths with per-name lookups (a new Backend::lookup_at, overridden by the in-memory and tar backends) instead of listing and merging whole directories, while still reading each mutable upper directory as one snapshot; the tar backend memory-maps the archive to build its index and reads file contents with positional reads; broker requests are served by a lazily started blocking-handoff worker pool; the shared-memory transports copy directly to and from mapped ring and buffer memory and wake the peer only when it is waiting; runner callers take turns reading responses through a reader role instead of a dispatcher thread; shared-buffer slots are leased lowest-first; a locally closed call channel ends notification receipt cleanly instead of reporting an error; and the Linux shim gives the platform a huge-page hint (PageManagementProvider::advise_fill, MADV_HUGEPAGE on Linux userland) before eagerly copying file contents into a new mapping.

[github-actions]

🤖 SemverChecks 🤖 ⚠️ Potential breaking API changes detected ⚠️

Click for details

--- failure auto_trait_impl_removed: auto trait no longer implemented ---

Description:
A public type has stopped implementing one or more auto traits. This can break downstream code that depends on the traits being implemented.
        ref: https://doc.rust-lang.org/reference/special-types-and-traits.html#auto-traits
       impl: https://github.com/obi1kenobi/cargo-semver-checks/tree/v0.51.0/src/lints/auto_trait_impl_removed.ron

Failed in:
  type TarRo is no longer UnwindSafe, in /home/runner/work/litebox/litebox/litebox_broker_core/src/fs/tar_ro.rs:50
  type TarRo is no longer RefUnwindSafe, in /home/runner/work/litebox/litebox/litebox_broker_core/src/fs/tar_ro.rs:50

--- 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/v0.51.0/src/lints/method_parameter_count_changed.ron

Failed in:
  litebox_broker_transport::pending_calls::PendingCall::wait takes 0 parameters in /home/runner/work/litebox/litebox/target/semver-checks/git-ulitebox/d5df8f676ed8d814f7090697a6c9fd5973eeb366/litebox_broker_transport/src/pending_calls.rs:111, but now takes 1 parameters in /home/runner/work/litebox/litebox/litebox_broker_transport/src/pending_calls.rs:155

--- failure method_receiver_ref_became_owned: method receiver changed from immutable reference to owned value ---

Description:
A method's receiver changed from an immutable reference to an owned value. Ownership is transferred as part of the call, which is a breaking change.
        ref: https://doc.rust-lang.org/reference/items/associated-items.html#methods
       impl: https://github.com/obi1kenobi/cargo-semver-checks/tree/v0.51.0/src/lints/method_receiver_ref_became_owned.ron

Failed in:
  litebox_broker_transport::pending_calls::PendingCall::wait now takes Self, not &Self, in /home/runner/work/litebox/litebox/litebox_broker_transport/src/pending_calls.rs:155

--- failure type_mismatched_generic_lifetimes: type now takes a different number of generic lifetimes ---

Description:
A type now takes a different number of generic lifetime parameters. Uses of this type that name the previous number of parameters will be broken.
       impl: https://github.com/obi1kenobi/cargo-semver-checks/tree/v0.51.0/src/lints/type_mismatched_generic_lifetimes.ron
Failed in:
  Struct PendingCall (0 -> 1 lifetime params) in /home/runner/work/litebox/litebox/litebox_broker_transport/src/pending_calls.rs:141

[Weidong Cui (wdcui)]

Split into #1495 (overlay lookups), #1496 (tar archive mapping, stacked on #1495), #1497 (transport fast path), #1498 (lazy broker workers), and #1499 (huge-page hint); together they are tree-identical to this branch. Note that this PR's base numbers were measured against a binary that already included the overlay and tar changes; against ulitebox (d798f1d) the combined change takes python3 -c pass from 162 to 46 ms and node -e 0 from 304 to 84 ms.