Invert feature gating: required_features → missing_features

[ianks]

Previously adapters declared what they support (allowlist / default-deny). This caused silent test-skipping: a new spec with required_features that no adapter declared would silently never run anywhere. The 27 broken ToSDrop specs from caf34da went undetected for 4 months because ruby_drops wasn't in the reference adapter's allowlist.

What changed

Polarity flip:

• Specs declare features: — informational tags describing what the spec exercises (renamed from required_features)
• Adapters declare config.missing_features = [...] — an explicit denylist of capabilities they can't provide
• A spec runs unless its tags intersect the adapter's missing_features

Default is now run everything. Skipping requires a conscious opt-out.

Coverage check (new):

Added a coverage_check rake task, wired into CI, that verifies every feature tag used in the spec YAML is "covered" — i.e. not in every adapter's missing_features. This makes orphan tags a CI failure rather than a silent gap.

Coverage check passed: all 15 feature tags covered across 10 reference adapters.

Reference adapters updated — each now declares what it genuinely can't handle:

• liquid_ruby.rb: missing Shopify-specific features + activesupport + lax_parsing
• liquid_ruby_shopify.rb: empty (runs everything — full Shopify Ruby Liquid)
• liquid_c.rb: missing runtime_drops, ruby_types, etc.

Also includes the ToSDrop counter restore fix and the [](key) method needed for map filter tests.

Migration for adapters outside this repo

# Before:
config.features = [:core, :strict_parsing, :ruby_types]

# After (conservative — preserves current skip behavior exactly):
config.missing_features = [:lax_parsing, :activesupport, :shopify_filters, ...]

# After (ideal — only list what you genuinely can't handle):
config.missing_features = [:shopify_filters, :shopify_tags]