docs: demote DESIGN.md to a design record and move the semantic model

[vyuma]

Closes #10

The decision

The issue offered (a) rewrite DESIGN.md as the current specification, or (b) demote it to a design record and move the still-true semantic model out. This takes (b), for the reason the issue gives: an authoritative spec written before the code cannot survive, and the semantic model is the part worth keeping.

What changed

DESIGN.md — retitled to a design record, with a header that says plainly that it is not a specification, is not synchronized with the code, and loses to the code on any disagreement. The header carries a table naming what actually governs each topic.

Removed outright, because they could only misdirect:

• §10 repository layout — a 78-line Python source tree; none of those paths exist.
• §12 implementation roadmap and §13 issue-sized backlog — phases 0–6 of future work, all of it shipped.

Marked superseded, with the living document named:

• §7 rule catalog — does not match the shipped 92 (MLC001 is missing from it); docs/rules/ and src/rules/registry.rs govern.
• §8 CLI/config/suppressions — missing shipped flags such as --analysis-summary; docs/reference/, docs/guides/configuration.md, schemas/, and qual --help govern.
• §11 test strategy — the fixtures are not named branches.py / expected.json, and the §11.4 release gate (200 hand-labeled cases, precision 98%, 95% Wilson lower bound 95%) is computed by no code; tests/corpus_gate.rs and the CONTRIBUTING labeling protocol are what is enforced.

Corrected against the code:

• The fix-validation section promised ast.parse(feature_version=...), contradicting the README and src/reporting/fixes.rs. It now states what the Rust implementation does: a fixed rustpython-parser 0.4 / Python 3.12 grammar with no feature_version pin, and target-python as a post-parse MLC000 gate.
• The supersession list claimed MLR112 > generic portability, which does not exist, and omitted the three shipped ones. It now names the six real relations and points at RuleMetadata::supersedes.

docs/architecture.md — gains The Manim semantic model in English: Scene lifecycle, the exact Scene.play state machine, frame time and the dt-parameter updater convention, membership/family/draw order, 3D fixed objects, and the renderer-specific point layouts. Legacy section numbers are kept in the headings (### Scene lifecycle (§3.1)), so the ~15 existing docs/rules/*.md citations of "DESIGN §3.x" still resolve — the intro explains the mapping.

README.md, CONTRIBUTING.md, AGENTS.md, docs/index.md, docs/releasing.md, docs/rules/README.md — now point at documents that match the repository. AGENTS.md is rewritten: no "start with Phase 0", no "do not claim a reserved rule is implemented" (zero are reserved), and the standing rules state that the catalog is finished.

Language decision, recorded in CONTRIBUTING.md, AGENTS.md, and the DESIGN.md header: authoritative documentation is written in English. DESIGN.md stays Japanese as the one deliberate exception, because a design record is preserved rather than maintained — no contributor needs to read it.

Acceptance

• No document that describes a non-existent Python implementation is labeled authoritative
• README.md and CONTRIBUTING.md point contributors at something that matches the repository
• The specific contradictions listed in the issue are corrected or removed
• AGENTS.md reflects a finished catalog
• A decision is recorded on whether the authoritative document is English

Verification

cargo test green (docs-only change, but the schema/golden tests cover the contracts referenced). Every relative link target checked to exist from the file that contains it.

🤖 Generated with Claude Code

[Hosi121]

Agreed with (b). A specification written before the code cannot survive, and the semantic model is the part worth keeping. Deleting the 78-line phantom source tree (§10) and the already-shipped roadmap (§12/§13) is straightforwardly right — those could only misdirect a reader.

Two coordination notes with #21:

• Both PRs append to CHANGELOG.md under [Unreleased], so the second to land needs a trivial rebase.
• #21 adds three new code comments citing DESIGN §8.2 honesty (in src/application.rs and src/config/loader.rs), and this PR marks §8 superseded. The principle is right and worth keeping in the code, but whichever merges second should repoint those citations at the living document — docs/guides/configuration.md, presumably — so a reader following the reference does not land in a section that says it loses to the code.