Give emoji-only post titles distinct filenames (#52)

[jmacdotorg]

Closes #52.

Diagnosis

Plerd::Post::_build_published_filename builds a slug from the title by stripping HTML, turning whitespace into hyphens, and then deleting everything that isn't \w or -:

$filename =~ s/[^\w\-]+//g;

A title made entirely of emoji (or any non-word characters) is reduced to the empty string, so the filename becomes YYYY-MM-DD-.html. Two such posts on the same day — e.g. 😀 and 🎩🎩 — both resolve to 2021-01-19-.html, and one silently overwrites the other on disk.

Fix

When the slug comes out empty, fall back to a short hash of the title:

unless ( length $slug ) {
    $slug = substr( md5_hex( encode_utf8( $self->title ) ), 0, 10 );
}

Distinct titles now get distinct, deterministic filenames (2021-01-19-2a02eac39d.html, 2021-01-19-0c35dfec4f.html), and the value is stable across republishes because it's written back into the source's published_filename metadata on first publish. md5_hex/encode_utf8 come from core Digest::MD5 (already a Plerd dependency) and Encode — no new deps. (Two posts sharing an identical title on the same day still collide, but that's the pre-existing, separate collision class that affects ordinary titles too.)

Test

Adds t/emoji_title.t: publishes two emoji-only titles on the same day and asserts the two published filenames are distinct, non-empty, and that both HTML files actually exist on disk. Failed before the fix, passes now. Full suite green (83 tests).

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