feat(csharp)!: verify strict-by-default with --lax escape (complete #96)

[dmealing]

What & why

dotnet meta verify loaded metadata lax, so an unregistered or typo'd own
@attr silently passed verify in C# while Java's force-strict Maven goal and
the strict TS/Python verify CLIs (#101) reject it — a port-dependent provenance
verdict (ADR-0023). This completes the C# half of #96.

Port	verify default
Java	strict (Maven goal LoaderOptions.create(false,false,true))
TypeScript / Python	strict-by-default + --lax (#101)
C# (this PR)	strict-by-default + --lax

The fix

• Thread strict through both verify load sites — VerifyCommand.Run
  (templates gate) and the --codegen gate — via a new Options.Strict
  (default true). The convenience MetaDataLoader.FromDirectory gains a
  strict param (default false / lax), so gen / docs / agent-docs keep
  loading lax — only verify defaults strict.
• Add a --lax flag to verify arg parsing (sets strict=false) to restore
  the legacy open-attr load.
• On a strict ERR_UNKNOWN_ATTR failure, print an actionable three-exit hint
  (register the attr on a provider / move it into an attr.properties bag / pass
  --lax). The loader error code and text are unchanged — only the command
  output gains the hint.

Before / after

# metadata with a made-up @attr on a field.string
$ dotnet meta verify <dir> --templates <tpl>          # before → exit 0 (silent)
$ dotnet meta verify <dir> --templates <tpl>          # after  → exit 1, ERR_UNKNOWN_ATTR + hint
$ dotnet meta verify <dir> --templates <tpl> --lax    # after  → exit 0 (legacy, opt-in)

Test evidence (TDD)

MetaObjects.Cli.Tests/VerifyStrictTests.cs — a metadata dir with one made-up
@attr:

• strict-by-default → ERR_UNKNOWN_ATTR load error, non-zero exit (templates and
  codegen gates);
• --lax → no load error, passes.

RED (new Options.Strict / Run(..., strict:) API absent) → GREEN.

Suites run (no Docker):

• MetaObjects.Cli.Tests — 42 passed
• MetaObjects.Conformance.Tests — 666 passed
• MetaObjects.Codegen.Tests — 242 passed, 1 skipped (opt-in regen fixture)
• MetaObjects.Render.Tests — 290 passed

CLI smoke: strict → exit 1 + hint; --lax → exit 0. No repo-own fixture
surfaced an unregistered attr under the new strict default. MetaObjects.IntegrationTests
(Testcontainers Postgres) not run — requires Docker.

Part of #96.

🤖 Generated with Claude Code