spp_demo_phl_luzon: wire the Luzon area/population loaders into demo tooling

[gonzalesedwin1123]

Follow-up from the #277 review (finding 2): spp_demo_phl_luzon ships spp.demo.luzon.area.loader and population-weight helpers, but no runtime path invokes them — installing the module does not populate Luzon areas (this matches the original #76 behavior; the DESCRIPTION has been corrected to say so).

The misluzon CLI profile (spp:137) installs the module alongside spp_mis_demo_v2, so the natural integration is for the demo generation flow (or a dedicated hook/wizard) to invoke:

• env["spp.demo.luzon.area.loader"].load_luzon_areas() (after spp.demo.area.loader.load_country_areas("phl"))
• the population-weight helpers for registrant distribution

Design questions for the implementer: post_init_hook vs generator integration vs explicit wizard; interaction with --generate; idempotency is already covered by the loader's pre-link strategy (tested).

Uninstall behavior verified safe on 19.0 (dual-owned areas survive; luzon-only areas cleaned up).

[gonzalesedwin1123]

Status update: the wiring this issue asks for already exists in draft #282 (the spp_mis_demo_v2 re-land) — its demo generator conditionally invokes spp.demo.luzon.area.loader.load_luzon_areas(load_shapes=True) when spp_demo_phl_luzon is installed, and consumes the population weights for registrant distribution. Original #76 had integrated the loaders through the demo generator; the caller simply lives in the Wave-5 module not yet merged.

Do not implement this independently — it would duplicate #282.

Closing checklist once #282 merges:

1. Verify ./spp start --demo misluzon --generate populates the 821 Luzon areas end-to-end.
2. Update spp_demo_phl_luzon/readme/DESCRIPTION.md (currently says wiring is 'planned as a follow-up') to state the demo generator invokes the loader when the module is installed.
3. Close this issue referencing feat(spp_mis_demo_v2): demo GIS reports/statistics data (re-land from #76) #282.