Planning v4.1: record the v1.0.0 release, and plan v1.1

Documentation only. No product code, requirement, acceptance test or
schema changes.

Post-release correction. v4.0 was written before the closeout commit was
signed (432f041), main was fast-forwarded to it, and the signed v1.0.0 tag
was pushed. Current-state wording now says so in README.md,
planning/README.md, BUILD-MILESTONES.md and VERSION.md. BUILD-MILESTONES.md's
header had been stale since M8. The M11 report is not edited: its §T
records the state at closeout.

v1.1 plan. planning/V1.1-PLAN.md triages the post-v1 backlog and the other
recorded v1 residual risks, and orders them into work packages, not
milestones:
- A1: a context-window safety reserve, plus reporting a turn the server
  truncated
- A2: removing protocol echoes from stored narration, and a genre-neutral
  state rule
- B: long-term memory retention that holds without help from state
- C: browser coverage of Retry, Save Points, correction, length, failure
  and export download
- D: integrity_check on backups, and a warning when an export exceeds the
  import limit
- E: WCAG 1.4.11 control-boundary contrast

Scheduled backups, the import limit, identity detectors and duplication
suppression move to v1.2; media adapters are future work. The first brief
to write is A1.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01VvegagkhuCZoFPdv4M1egY

README.md

@@ -346,9 +346,11 @@ most interesting engineering in the repo.
 
 ## Repo notes
 
-- **Status:** milestones M1-M11 are complete. The v1 release gate passed on
-  2026-09-14 (see [`planning/reports/M11-IMPLEMENTATION-REPORT.md`](planning/reports/M11-IMPLEMENTATION-REPORT.md),
-  §T). Passing the gate is not a release: there is no `v1.0.0` tag yet.
+- **Status:** **v1.0.0 was released on 2026-09-14.** Milestones M1-M11 are
+  complete, and the v1 release gate passed (see [`planning/reports/M11-IMPLEMENTATION-REPORT.md`](planning/reports/M11-IMPLEMENTATION-REPORT.md),
+  §T). The signed tag `v1.0.0` and `main` both point at the signed release
+  commit `432f041`. v1.1 development has begun on the `v1.1-development` branch;
+  its plan is [`planning/V1.1-PLAN.md`](planning/V1.1-PLAN.md).
 
 - `planning/` is this fork's own package: the product specification, the architecture
   decisions, the milestone plan, the acceptance contract, and a review report for every