Promote WASI 0.3 to the primary preview (#929)

* Promote WASI 0.3 WIT to canonical and collapse 0.3.0-rc/draft to 0.3.0

Remove the 0.2 wit/ directories, rename wit-0.3.0-draft/ to wit/ for each
proposal, update deps paths, and collapse 0.3.0-rc-2026-03-15/0.3.0-draft
version identifiers to 0.3.0.

Update documentation to reflect WASI 0.3:
- capture WASI 0.3 inclusion criteria in Proposals.md
- treat WASI 0.3 streams as available, not upcoming
- replace wasi:http/proxy world with service and middleware
- remove obsolete streams workaround and wasi:io references
- rewrite POSIX compatibility for 0.3 native async

* ci: drop WASI 0.2 and RC paths

- ci.yml/validate-proposals.js: single wit/ filter (WIT_FILES)
- release.yml/release.sh: single 0.3.x release flow
- always regenerate proposal markdown docs from wit/
- publish.yml/publish-proposal.yml: publish from wit/, drop io and the
  wit-0.3.0-draft/prerelease branches
- RELEASE.md/scripts/README.md: update docs to the 0.3 flow

* fix: code review suggestions

Release WASI v0.2.12

Release WASI v0.2.11 (#907)

Co-authored-by: ricochet <974323+ricochet@users.noreply.github.com>

Release WASI v0.3.0-rc-2026-03-15 (#904)

Co-authored-by: ricochet <974323+ricochet@users.noreply.github.com>

Release WASI v0.3.0-rc-2026-02-09 (#882)

Co-authored-by: ricochet <974323+ricochet@users.noreply.github.com>

Release WASI v0.2.10 (#878)

Co-authored-by: ricochet <974323+ricochet@users.noreply.github.com>

Release WASI v0.3.0-rc-2026-01-06 (#866)

Co-authored-by: ricochet <974323+ricochet@users.noreply.github.com>

Release WASI v0.2.9 (#841)

Co-authored-by: ricochet <974323+ricochet@users.noreply.github.com>

Release WASI v0.2.8 (#670)

Co-authored-by: ricochet <974323+ricochet@users.noreply.github.com>

Release WASI v0.2.7 (#668)

Co-authored-by: ricochet <974323+ricochet@users.noreply.github.com>