docs(release): prepare v0.2.0 rehearsal and owner handoff

[codeforester]

Scope

Preparation for #303 only; intentionally does not close the human-owned publication issue. Document the exact stable versus advisory input boundary, three-working-day minor-release bake, review/waiver decisions, final-candidate evidence, fresh/upgrade rehearsal and downloaded-asset checks. Correct the procedure to check the finalized external BOM rather than requiring the deliberately uncertified tracked record to pass.

Prepared installer tests now compare self pins with the prepared BOM instead of requiring the future demo tag to exist during its version PR. Released Base tags still resolve exactly; final demo tag identity and compatibility remain enforced by provenance and live evidence. Add a regression showing 0.2.0 can be prepared without creating a tag or certifying untested evidence.

Validation

• Eight BOM/evidence unit tests, nine release BATS, nine source-installer BATS and nine exact-public-input BATS passed; metadata/dependency checks and git diff --check passed.
• Live read-only mechanics rehearsal at main 44621c1 using successful push run https://github.com/basefoundry/base-demo/actions/runs/35365709780: finalizer generation/reverification, strict live-evidence BOM gate, both SHA-256 manifests and nine installer fixtures passed.
• This rehearsal uses the still-current 0.1.0 metadata solely to test mechanics. It is not a final 0.2.0 candidate or replacement release and was not published.
• Full local validator executed all four live HTTP contracts then stopped at the existing Xcode license requirement; all seven hosted checks passed at a4f2fab: https://github.com/basefoundry/base-demo/actions/runs/35368336767.

Remaining human-owned work

Version-preparation change, final exact candidate, fresh actual install/upgrade rehearsal, bake interval, review or explicit waiver, publication permission, annotated tag/release, downloaded verification and final public URL/digest. No release assets or historical versions modified. Builds on merged #316. The net change against main is five release-preparation files. The computed merge tree is de664114eb853724e68777129b23242c2bcf66a0, exactly matching the seven-checks-green head; no source changes or rebase were needed.