Stop the arctic backend from dragging std in, and 0.1.5

[pathscale]

Stacked on #5, which is stacked on #4. Merge order is #4, #5, then this one.

The bug this fixes

worktable-vec declares #![no_std]. Enabling the arctic feature enabled arctic's default features, which are std plus an SMR backend, so the crate linked a standard library it says it does not use. Nobody noticed because it was only ever built on a host that has one.

arctic 0.1.11 is the first release where this is fixable at all: until pathscale/arctic-wt#9 landed today, smr-ps-reclaim forced std on by itself, so no_std + reclamation was not a combination that existed.

--features arctic        arctic-wt [smr-ps-reclaim]      ps-reclaim [libc,spin]
--features arctic,std    arctic-wt [smr-ps-reclaim,std]  ps-reclaim [libc,spin,std]

A std feature now exists and forwards; it is off by default, because the core never needed one.

congee and wti gained "std" in their feature lists. Neither crate is no_std upstream — congee-wt has no #![no_std] at all and WorkTablesIndex's concurrent feature pulls parking_lot — so the manifest now says so instead of failing later at link time.

What is actually no_std, precisely

build	x86_64-unknown-none
core, no features	builds
hydrate	builds
arctic	no — needs an OS
congee, wti	no — std crates upstream

The arctic row is not a defect to fix here. ps-reclaim keeps its participant slot in thread-local storage, and its no_std path is pthread keys, so it needs an operating system. no_std means no standard library, not no operating system, and the distinction is now written down rather than assumed. A bare-metal reclamation backend would need a slot mechanism that does not exist yet; that is a ps-reclaim question, not this crate's.

The verifier

A rule with no check is a comment. CI grows three steps inside the existing job — no new runner:

• core and hydrate build for x86_64-unknown-none, a target with no std to find, so the claim is checked against a target instead of a host
• the arctic dependency graph is asserted to carry no std feature

The assertion was run against --features arctic,std first and it failed there, which is the only evidence that it can fail at all.

Checks

cargo test --all-features                    31 passed, 0 failed (+3 ignored disk tests)
cargo test --no-default-features --features arctic   10 passed, 0 failed
cargo clippy --all-features --all-targets    clean
x86_64-unknown-none, core and hydrate        builds

Version 0.1.5, covering #4, #5 and this — one publish for the series rather than three.

[pathscale]

Folded into #4. Same commits, no new branch.