[shinaoka]

Release preparation for threadteam 0.2.0: the version bump and an "Upgrading from 0.1" section in the README. This PR does not publish; publishing waits for the maintainer's approval.

Why 0.2.0 (not 0.1.4)

cargo public-api -p threadteam diff v0.1.3..origin/main, default features and --all-features.

Signatures: additive.

• Added: PoolBuilder::spin_polls, Pool::spin_polls and, with the rayon feature, Exec::external_rayon.
• Nothing threadteam declares was removed or changed.
• With default features, the blanket impls that came with Rayon's dependencies (either::IntoEither, crossbeam_epoch::Pointable on every public type) disappear, because parallel no longer pulls Rayon.
• Pool and IntoTeamError lose the auto trait Freeze, which is unstable and not nameable on stable.

Behaviour: breaking. That makes it a minor bump under 0.x semver:

change	effect on callers
Pool drop with default thread settings	the shared workers are returned to the process-wide reserve, not joined. Dedicated pools (thread_name, on_start, stack size, QoS) are still joined
external on a team token	lends the members to the pool's own queue; Rayon calls in the callback now go to Rayon's global pool. The old behaviour is external_rayon (feature rayon)
feature parallel	no Rayon dependency
idle_runs_rayon	no effect, kept so that callers compile
waits	libgomp's (spin_polls, 16 yields, sleep)

Package check

• cargo package packages and verifies threadteam v0.2.0: 113 files, about 1.2 MB, the same layout as 0.1.3, with no include/exclude list.
• The metadata is unchanged from 0.1.3: name, description, license MIT OR Apache-2.0, repository, documentation, readme, keywords and categories.
• rust-version is not set, as in 0.1.x.
• Local checks pass: fmt, clippy (all features and no default features), doctests (74) and cargo doc -D warnings. The full gate and the acceptance runs (broccoli (a)(b)(c), evo (b)(d)) ran on Own worker threads: a process-wide reserve of std::thread workers instead of a private Rayon pool (#15) #16, whose head is this branch's parent 9eeb0b4.

After approval (not in this PR)

1. Publish 0.2.0 and tag v0.2.0.
2. Raise faer's threadteam requirement to 0.2, then the tenferro v0.8.0 chain.

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