Expand conformance suite to per-rule fixture coverage, gated (herb-embedded-ag7)

[jleo3]

Summary

• Adds one hand-written .html.erb fixture per rule the vendored bundle registers (100 rules — verified as the true, complete count against @herb-tools/linter's src/rules/, which has 106 files but 6 are shared utility modules, not rules) under spec/conformance/fixtures/rules/. Each was authored by reading the rule's TypeScript source and validated to trigger the rule via the real reference linter CLI (node_modules/.bin/herb-lint --only <rule>).
• conformance_spec.rb's existing reference_offenses/bridge_offenses helpers gain optional only:/rules: args (extended, not duplicated) so a per-rule fixture can force-select just that rule the same way gu7/ada's specs already do — this also exercises rules not enabled by default. A new coverage-gate test enumerates bridge.rule_names and asserts every one has a fixture, so a future upstream rule with no fixture fails the build instead of going untested silently.
• Three rules that only enable themselves for partial files (basename starting with _) get underscore-prefixed fixture filenames to trigger correctly.
• Found and fixed a real bridge bug while running the new suite: __herbLint/__herbAutofix in js/ruby_backend.js construct the vendored Linter without its 4th constructor arg (allAvailableRules), which silently defaults to just the currently-selected rule subset. herb-disable-comment-unnecessary depends on that arg (via context.validRuleNames) to recognize other rules referenced in a herb:disable comment — under our one-rule-at-a-time execution model it could never recognize any rule but itself. Fixed by passing HerbLinter.rules explicitly at both call sites.
• CHARTER.md's "conformance punch list isn't closed" open question is updated to reflect the fixture-coverage dimension is now closed, without overclaiming the separate .herb.yml config-application item it also named.

Acceptance criteria

• One .html.erb fixture per built-in rule under spec/conformance/fixtures/rules/, hand-written, each verified to trigger its target rule via the real reference linter.
• A new coverage-gate test enumerates every rule the vendored bundle registers and asserts each has a fixture — fails the build if any rule is uncovered.
• All new fixtures pass the existing offense-for-offense diff against the reference linter, reusing (not duplicating) reference_offenses/bridge_offenses.
• bundle exec rake passes, including the expanded conformance suite: 206 examples, 0 failures, 0 RuboCop offenses.
• CHARTER.md updated to match, without adding scope beyond what this work actually changes.

Test plan

• bundle exec rake passes locally (206 examples, 0 failures).
• Full conformance suite alone: 107 examples (100 rule fixtures + 5 legacy fixtures + minimum-fixtures check + coverage-gate), 0 failures, ~16-20s.
• Independently verified exactly 100 fixture files, all unique, 1:1 mapped onto bridge.rule_names with no extras and no gaps.
• Verified the herb-disable-comment-unnecessary fix by re-running the full suite before (1 failure) and after (0 failures) the js/ruby_backend.js change.

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