Fix specialty keytags showing by default when the setting is off (#24)

[dgershman]

Closes #24

Problem

On a fresh install, the specialty (over-2-years) keytags render on the frontend even though Settings → NACC → Show Special Keytags is unchecked and the label says "Default is false."

The settings UI and the frontend disagreed on the default:

• DEFAULT_SHOW_SPECIAL was the string 'true', and both render paths read get_option( 'nacc_special', self::DEFAULT_SHOW_SPECIAL ). The 2nd get_option() arg only applies when the option row is absent (any install where the form was never saved), so the JS constructor received a truthy "true" and specialty tags rendered.
• register_settings() registered nacc_special without a 'default' key (unlike nacc_theme/nacc_language/nacc_layout), so get_option('nacc_special') in draw_settings() returned false and the checkbox rendered unchecked.

Net: checkbox off, frontend on.

Fix (scoped to the default-state bug)

• DEFAULT_SHOW_SPECIAL → boolean false.
• Register 'default' => false on the nacc_special setting so draw_settings() and the frontend get_option fallback agree.
• Normalize the value handed to the JS constructor to a real JSON boolean: wp_json_encode( (bool) $args['special'] ) in render_keytags() (shared by both the shortcode and legacy nacc_content() paths). This also removes the stringified "0"/"true" truthiness trap flagged in the ticket — the JS side does a truthy check (nacc2/src/nacc.js:59), and a real JSON false renders no specialty tags.

No JS source change was needed (the constructor already treats the 4th arg as a boolean/truthy check), so the shipped bundle is unchanged.

Verification

• Fresh/unsaved install (no nacc_special row): get_option('nacc_special', false) → registered default false → wp_json_encode( (bool) '' ) → JS false → no specialty tags, and the checkbox reads the same false default → unchecked. Consistent.
• Box checked + saved: option stored true → JS true → tags show; checkbox checked.
• php -l clean; phpcs (.phpcs.xml) passes with no findings.

Shared version / readme bump (see #24/#25/#26)

This is the base of the #24/#25/#26 release set. Per the ticket comment, the version bump and readme.txt changelog are meant to be managed once for the combined set. Since #25/#26 aren't merged yet, this PR makes the smallest correct change: version 6.0.1 → 6.0.2 (plugin header + Stable tag) and a single changelog entry. Reviewer/mergers: please treat the version + changelog as shared with #25/#26 and avoid double-bumping when those land — fold their notes into the same 6.0.2 entry rather than adding another bump.