docs: reorganize reference/, add library reference docs, remove POC

[ModifAmorphic]

Housekeeping across docs/ + AGENTS.md. Docs only (the two enginseer/discovery changes are comment-only).

What changed

Reference re-org — docs/reference/ is now categorized:

• darktide/ — darktide-binary.md (game-binary facts)
• community-tools/ — darktide-framework-analysis.md + analysis-verification.md
• magos-modificus/ — NEW per-library API reference (7 docs: general, config, profiles, shared-mods, steam, integrations, enginseer-client), authored from the code (public interfaces/methods, key types, DI registration, cross-platform notes). Index README rewritten.

Architecture — MAGOS-MODIFICUS.md: new Composition & startup section (composition root, Add<Library>() chain + ordering, startup sequence, TryAdd DI contract); status + project-layout updated to Phases 0–2; the broken DMF-sourcing claims rewritten as the open Phase-4 decision.

POC removal — deleted docs/poc/ (9 files). The load-bearing content was already distilled into docs/reference/darktide/ + docs/architecture/ (verified: every POC "unproven/risk/must-verify" item is either resolved by the built runtime or tracked as future work). Cleaned all docs/poc link leakage across AGENTS.md, the architecture docs, docs/reference/, the root README, and two enginseer/discovery Rust comments that cited the deleted production-spec. The poc branch refs in AGENTS.md are intentionally kept — only the docs folder is removed.

Doc currency — stale status fixed everywhere (Phase 0 → Phases 0–2; integrations/steam/enginseer-client "stub" → implemented) across AGENTS.md, root README, magos-modificus/README, architecture/README + MAGOS-MODIFICUS. AGENTS.md tests list completed; per-library reference-doc maintenance rule added to the doc-currency section.

Validation

• Code-review: every one of the 7 new reference docs was verified line-for-line against the code (public signatures, DI registration, key types, cross-platform behavior); the Composition section matches MagosComposition.cs exactly; re-org consistency confirmed (no residual docs/poc or old reference paths; all anchors resolve). Two findings folded in (root README status miss; two dangling Rust comment citations).
• Repo-wide sweep: zero remaining docs/poc references in code or docs.

Decisions made (redirect if wrong)

• Composition as a section in MAGOS-MODIFICUS.md (not a separate doc).
• analysis-verification.md placed in community-tools/ (it also audits darktide-binary, minor).

Not in this PR (flagged for follow-up)

• Two genuinely-open risks surfaced from the POC audit — worth the issue tracker, not docs: AV false-positive mitigation (distribution concern) + Fatshark flipping on client-side EAC (accepted risk).
• Four minor code observations the reference-doc pass turned up (separate PRs if wanted): config.example.json values are literal <app-data>/... placeholders (a footgun — an empty file is safer than the example verbatim); a stale Program.cs path in the enginseer-client.Tests csproj comment; an unused DarktideSteamAppId const; Steam is the one backend library that doesn't reference MagosConfig (intentional, asymmetry only).