Record the stable 1.0.0 release

[hokoo]

Outcome

Record the completed, independently verified stable 1.0.0 release and close
the 1.0 delivery plan.

Closes #24.

Recorded evidence

• Preparation PR #38: exact head
  8694c9e27adcd90f384a98c52a22b0c4dfdc51c1, required CI
  34625148206,
  merge a0437fcf6e8c1ff5644161f2dc265b4e66d309b5, post-merge CI
  34625310223,
  all 5/5.
• Non-mutating Release run
  34625517011:
  exact approved tuple intent and five gates passed; publication skipped;
  209-byte notes SHA-256
  057ad95f9a44421bf24dd795d8a39bf132daac1ce19fa44b7fe5a754a47c2fa3;
  tag and Release remained absent.
• Owner-approved publication run
  34626407305:
  every gate reran and passed before the sole mutation job.
• Immutable stable/latest
  Release 1.0.0:
  tag and target equal exact a0437fc…; notes match; RC remains immutable
  prerelease.
• Packagist source/dist refs and both exact 1.0.0 and normal ^1.0 clean
  installs equal the approved SHA and pass the eight-entry, 19-method,
  installed-provenance, private-seam, and no-dev checks.
• Independent prepublication and post-release E5 reviews found zero blocker,
  high, medium, or low findings.

Changes

• Mark E5, T15, Batch 8, and the exact publication gate completed.
• Add the immutable release, CI, Packagist/install, and independent QA evidence
  to the register and transition log.

This PR changes only docs/plan-1.0.md. It does not change or republish the
immutable tag/Release, runtime, API, dependencies, tests, workflows, or package
policy.

Verification

• git diff --check: PASS.
• make check: PASS — 70 tests / 970 assertions.
• make test.package-install: PASS — Git and Composer production archives,
  eight roots, 19 methods/private seams, installed provenance, zero dev.
• Independent post-release E5 QA: PASS, no findings.

After protected merge, #24 will close; the 1.0.0 milestone can then be closed
after a final issue/release/Packagist read-back.