Modernize placeholder integration (PlaceholderAPI, component-native)

[minoneer]

Closes #134 (placeholder provider/consumer modernization; the chat-delivery follow-ups from that issue are tracked separately below).

Summary

Replaces the MVdW-era String placeholder pipeline with a component-native system, designed from scratch (spec-driven):

• Core placeholder package: PlaceholderSource (the 26 island values as Adventure Components, any-thread contract), PlaceholderService (MiniMessage TagResolver composition — placeholders are tags now), PlaceholderIntegration SPI discovered via ServiceLoader.
• <usb:KEY> tags in island/party chat formats, always available; unknown keys render literally without degrading the message. Legacy {usb_*} tokens in user configs migrate automatically (config v124).
• New uSkyBlock-PAPI module (FAWE-style, shaded, me.clip isolated): bundles the uskyblock PlaceholderAPI expansion — all 26 placeholders as %uskyblock_*%, registered automatically, superseding the dormant eCloud expansion — and contributes the <papi:PLACEHOLDER> tag so other plugins' placeholders render in our chat formats (legacy-deserialized, never MiniMessage-parsed: injection-safe).
• Caching redesigned: the uniform 20s cache is gone; only the genuinely expensive creature counts are snapshotted — per island, main-thread-confined, async callers get stale-or-pending. The old design ran main-thread-only entity scans on whatever thread hit a cache miss.
• ChatLogic renders components end-to-end — the first sendLegacy call site removed; legacy output stays byte-identical (pinned by test).
• Deleted: MVdW hook + dead be.maximvdw dependency and its dead repo (this makes the build cold-cache resolvable again — same rot that broke CI on Harden world loading against missing void generator #142), the {usb_*} regex engine, both flag-gated listeners (one was a no-op bug), the placeholder.* config flags.

Notes

• 49 new unit tests; 263 total green across modules. E2E-verified on Paper 1.21.11 + PlaceholderAPI 2.12.2: expansion registers, config live-migrates 123→124, <usb:...>/<papi:...> resolve. The e2e caught (and this branch fixes) a lifecycle bug: integrations now enable in delayed-enable, after POSTWORLD plugins exist.
• /usb reload re-registers the expansion; PAPI 2.12.2 replaces same-identifier internal expansions (verified against its bytecode), so no duplicate/stale registration.
• Shares the Maven-repo-fix commit with Harden world loading against missing void generator #142 (dedupes on merge, either order).
• Post-merge maintainer action: archive uSkyBlock-Expansion with a README pointer to the bundled expansion.
• Follow-up (tracked in Modernize placeholder integration and move chat delivery to components #134's remaining scope): the other 8 sendLegacy call sites and the bukkit-utils command framework's legacy surface.

🤖 Generated with Claude Code

[minoneer]

Live in-game verification completed on the dev server (Paper 1.21.11, PlaceholderAPI 2.12.2): island chat with format [L<usb:island_level>|R<papi:uskyblock_island_rank>] rendered [L1.9|R1] after /is level + /is top — confirming the internal <usb:...> tag, the full PAPI round-trip (<papi:...> → PAPI → bundled uskyblock expansion → back), the unpopulated-data fallbacks (0/N/A on a fresh island), and component-native chat delivery, all live.

[minoneer]

Entity-count placeholders also verified live: /papi parse me %uskyblock_island_monsters% / %uskyblock_island_animals% returned correct live counts with the 10s snapshot TTL behaving as designed (stale value within the window, refreshed after).