Add planet environment-extra materials to capex calcs; buildcost.py tool

Every building needs a planet-hazard material bundle on top of its normal
BuildingCosts (SEA for low pressure, HSE for high pressure, MGC for low
gravity, BL for high gravity, INS for cold, TSH for heat - area-scaled
where applicable), confirmed against live in-game build panels on Nike.
FIO's own BuildRequirements field is an unreliable superset across hazard
types, not a direct per-building list - don't use it directly.

- puga/planet.py::environment_extras(): the real threshold table.
- tools/buildcost.py: full construction material list for one building,
  optionally on a given planet.
- Wired into whatif.py/buy.py (via simulate(planet=...)) and scan.py's
  own bcost(), so their capex numbers stop silently omitting this.
- docs/mechanics.md: the rule, plus a separate forum-sourced note on the
  CX order-cancellation penalty.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

docs/roadmap.md

@@ -4,6 +4,9 @@ Open work only - what's already built/fixed is discoverable from `CLAUDE.md`'s T
 `docs/library.md`, `docs/decisions.md`, and module docstrings, not repeated here.
 
 ## Todo
+- [ ] `tools/network.py`/`puga/network.py`'s `combine()` doesn't pass `planet` into `simulate()`
+  yet (environment-extra construction materials, `puga/planet.py::environment_extras()`) -
+  `whatif.py`/`buy.py`/`scan.py` already do.
 - [ ] `tools/arb.py`: inter-CX arbitrage at real fill size (replaces `prun_cxarb.py`, which
   overstated profit 7-11x on 2/4 top hits).
 - [ ] `tools/haul.py`: fuel cost / profit per trip (route.py has the jump graph).