Prepare shuttle 0.9.4 release

[sarsko]

Cuts 0.9.4 for everything that has landed on main since 0.9.3: the parking_lot RwLock upgrade fix and the BatchSemaphore::upgrade change under it (#351), the portfolio panic-in-drop fix (#346), the two performance changes (#321, #322), the blocked-future backtraces (#215), and the annotation feature fix (#334).

shuttle goes 0.9.3 -> 0.9.4, shuttle-engine and shuttle-std go 0.1.1 -> 0.1.2, and shuttle-parking_lot-impl goes 0.1.1 -> 0.1.2.

#346 had no changelog entry, so this adds one; #321 and #322 were missing their PR references. #320 is test-only and gets no entry.

shuttle's own source is untouched this release — every fix is in shuttle-engine, shuttle-std or shuttle-parking_lot-impl — so raising the requirements to 0.1.2 is the whole content of the shuttle bump. It is also what makes the bump meaningful: cargo prefers versions already in the lock file, so under a ^0.1.1 requirement a user moving to 0.9.4 with cargo update -p shuttle would keep shuttle-engine 0.1.1 and get a new shuttle containing none of the fixes. (A full cargo update would pick 0.1.2 up either way; the requirement is what rules out the 0.9.4-without-the-fixes combination entirely.)

Two crates deliberately stay where they are:

• shuttle-schedulers at 0.1.1. It has no changes since 0.1.1 and takes shuttle-engine as ^0.1.1, so what is already published builds against the new engine.
• shuttle-parking_lot at 0.12.5, so that it keeps mirroring the parking_lot version it wraps. It takes the impl as ^0.1.0 and re-exports it with a glob, so the RwLock fix reaches its users without a republish, the same way shuttle-tokio picked up shuttle-tokio-impl-inner 0.1.2.

shuttle-parking_lot-impl's requirement on shuttle is raised from <0.10 to >=0.9.4, <0.10: its RawRwLock now calls BatchSemaphore::try_upgrade, which does not exist before 0.9.4, so the old requirement admitted a combination that does not compile.

Patch rather than minor: the only public API change since 0.9.3 is the addition of BatchSemaphore::try_upgrade. BatchSemaphore::upgrade keeps its signature, and the remaining wrapper crates depend on shuttle via 0.9, <0.10 or *, so they need no changes.

cargo nextest run --release --workspace (673 passed), cargo test --release --doc --workspace and cargo check --workspace --all-targets are clean.

Publish order once merged: shuttle-engine, then shuttle-std, then shuttle, then shuttle-parking_lot-impl.

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