Make effective discovery the default for type/member -D; drop empty sections

[richlander]

Summary

Improves the discovery (-D/--discover) UX for the type and member commands so that discovery advertises what you can actually query, not a static superset.

Two related changes:

1. Drop empty sections from effective discovery + clearer messaging (88b4f22)

   • Effective discovery now reports only sections/columns that actually render for the given type (e.g. drops member-detail code sections and a Custom Attributes section that a type cannot populate).
   • A valid-but-empty section reports note: section '<X>' has no data for this type (exit 0) instead of the misleading Error: Section '<X>' not found.

2. Make effective discovery the default; add --schema opt-out (cbab5c0)

   • Plain -D/-D <Section> for type/member now defaults to effective discovery (resolve source, list only sections with data), fixing the footgun where the static schema advertised sections a type can never have (e.g. Custom Attributes).
   • New --schema flag opts back out to the cheap, offline static schema listing.
   • --effective is still accepted (now redundant for type/member); --effective --schema resolves to --schema with a warning (only during discovery).

Scope

• Limited to type/member (ApiOptions). package/assembly keep their existing static-by-default + --effective opt-in behavior, because their effective path runs a full-package inspection (cost/offline regression risk).
• The "no source" case for type/member cannot regress into an error: the parser early-returns a static listing before any source is required, so effective is only reached when a source actually exists.
• Cache-first: effective only triggers a NuGet download on a cache miss (the same cost the real query would pay), then caches it.

Behavior

Command	Before	After
type <T> -D	static schema (all sections)	effective (sections with data)
type <T> -D --schema	n/a	static schema (offline)
type <T> -D --effective	effective	effective (unchanged)
bare type -D (no source)	static	static (unchanged)
package <P> -D	static	static (unchanged)

Tests

• Added coverage: effective-by-default drops empty sections; --schema restores the full static list; --schema parses to opt out of effective discovery; bare/no-source stays static.
• Updated the static-path counterpart tests to pass Schema = true explicitly.
• Full suite green except 9 pre-existing environmental ILDisassemblerComparisonTests failures (missing ilspycmd).

Docs

• docs/design/schema-query.md updated for the new default + --schema.