feat(english): add Firebird's Nest source plugin

[RibatTRW]

Addition

New English source plugin for Firebird's Nest (firebirdsnest.org), a WordPress.com-hosted fan-translation site with no multisrc-theme fit. Adds plugins/english/firebirdsnest.ts and its 96x96 icon. No related issue.

• Novel list: the site has no novel browse — the homepage is a feed of chapter-announcement posts. The catalogue is the nav "Projects" sub-menus (5 live novels); page 1 returns those 5, page 2 returns [] because the catalogue genuinely is one page.
• Novel detail: title, Author:/Status: lines, link-free synopsis, and the hand-maintained table of contents from each novel's page entry.
• Chapters are child pages; dated posts are announcements linking to them (pre-2017 posts holding text directly are also handled). The site's own ToCs are stale (Heavy Knight's stops at v2-ch19 while publishing runs through v3-ch39), so the plugin merges the ToC with every announcement from the novel's paginated tag archive: 133 Heavy Knight chapters (45 v1 + 49 v2 + 39 v3). Announcement slugs resolve to real chapter pages, never served as chapters.
• Search uses native WordPress search mapped back to the novel catalogue.

Verification

check:plugin passes all four steps live (5 popular, 1 search result, 133-chapter novel, 18894-char chapter). All five novels parse clean (redo-prince 69, no-fatigue 58, hello-work 89, contractor 3, including cross-hosted collaborator chapters); search resolves each novel plus page 2 correctly. New files clean under eslint/prettier; build:compile passes (repo-wide lint/format findings are pre-existing on base).

Limitations: no genre tags or cover art on the site, so defaultCover is used; the catalogue follows the nav menus, so a novel absent from the menu would not appear (today that correctly excludes a dead 404 page linked only from /projects/). AI-assisted contribution, no human review.