v4: README refresh and v3 -> v4 migration guide

[thomhurst]

Problem

The README predates the v4 changes. After PR #332 (and the other v4 issues) it needs:

• A v3 → v4 migration section: eager input materialization (iterator exceptions now throw at build time; one-shot enumerables now supported), non-blocking synchronous Dispose(), removal of the 10,000-task/batch caps and 24h timespan cap, trimmed wrapper-struct surface, build-time validation of maxConcurrency/parallelism on all variants, .NET 8 minimum.
• Updated behavioural docs: worker-pool execution model for throttled/rate-limited/timed processors, disposal semantics (async waits bounded, sync does not block).
• The maxConcurrency vs levelOfParallelism story — rewritten once the overload consolidation lands.
• Support policy: net8.0 minimum, tested TFMs.

Acceptance criteria

• README examples compile against v4 (ties into the Example-project issue).
• Migration section covers every intentional breaking change shipped in v4.