feat(codegen): split sidecar DDL by dialect; reject json sidecar (#45)

[hyperpolymath]

Resolves #45 (V-L2-F1) — split overlay DDL by dialect; resolve the json sidecar.

What changed

• New codegen::overlay::SqlDialect { Sqlite, Postgres } + from_storage() mapping [sidecar].storage.
• generate_sidecar_schema(schema, octad, dialect) dispatches to overlay::sqlite::generate / overlay::postgres::generate. Portable table bodies are shared via assemble(); the only dialect-divergent fragment (metadata upsert) is per-module: SQLite INSERT OR IGNORE vs PostgreSQL INSERT … ON CONFLICT DO NOTHING.
• The SQLite-only datetime('now') replaced by portable CURRENT_TIMESTAMP (both engines).
• storage = "json" is now rejected with a clear error (it previously silently emitted SQLite DDL for a JSON store). Implementing/dropping the JSON store is split out as V-L2-F2: implement the JSON sidecar storage backend (split from #45) #112 (V-L2-F2); SidecarConfig docs updated.
• main.rs derives the dialect from the manifest.

Acceptance

• Per-dialect modules exist (overlay::sqlite, overlay::postgres)
• DDL tests pass for both SQLite and PostgreSQL fixtures
• json sidecar storage is resolved — rejected here, tracked separately by V-L2-F2: implement the JSON sidecar storage backend (split from #45) #112

6 existing overlay tests retained (dialect-explicit) + 5 new. Suite: 112 lib + 9 integration green. The single red is the pre-existing failing-by-design provenance_fork_test (#104, fixed by open PR #109) — on main, unrelated.

Note: developed in an isolated git worktree because a concurrent session was mutating the shared checkout; this branch contains only the #45 change (no #51/#38 contamination).

🤖 Generated with Claude Code

[github-actions]

🔍 Hypatia Security Scan

Findings: 21 issues detected

Severity	Count
🔴 Critical	2
🟠 High	8
🟡 Medium	11

⚠️ Action Required: Critical security issues found!

View findings

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