feat: generate the world map on the server

[JJiang28]

Summary

• Adds internal/world, a dependency-free procedural generator producing three orthogonal planes (ground, relief, vegetation) plus rivers and special resources, deterministic from constants.WorldSeed. The world was previously a featureless 75×75 grid with no terrain of any kind.
• Ships the map via a new MapService.GetTerrain, packed one byte per tile — a few KB for the whole map, so it is neither paged nor filtered by vision.
• Makes settlement placement terrain-aware: canPlace now rejects footprints on water, peaks and ice, and player starts are chosen by scanning and scoring the map rather than drawing random empty blocks.

Notes for reviewers

Two decisions that are load-bearing and easy to undo by accident:

• Thresholds are quantiles of the elevation field, not fixed cutoffs. Land is exactly landFraction regardless of what the octaves do, so retuning noise can't silently flood or drown the world.
• Noise is sampled at hexPoint(), not at (col, row). Columns sit 1.5 apart and rows only ~0.866, so grid-space sampling stretches every coastline and mountain range ~1.73× horizontally once drawn.

FindEmptyCityBlock was rewritten rather than tweaked. Towns are seeded first and take the good land, so whatever the SQL query leaves free is disproportionately the water and mountain the seeder just rejected — drawing from it seated the test player inside a mountain range. It now scans in memory and scores candidates on fertility, fresh water and coastline.

Terrain-aware rejection halved the towns surviving seeding, so townMinSpacing drops 5 → 4 to hold density roughly where it was. That is a gameplay-density change, not just a visual one.

Test plan

• go build ./..., go vet ./... and gofmt -l internal/ cmd/ clean
• buf lint clean; buf generate reproduces the committed output exactly
• Boots against a local Postgres and serves: 99 towns seeded, every one on buildable ground, no "no habitable start block" warnings
• Rebased onto main after feat: troops & armies (existence phase, no combat) #12 (troops & armies); both coexist — army actors spawn and terrain generates in the same boot
• Generation is deterministic (identical planes on regenerate) and every river link is reciprocal across the shared edge
• Distribution is stable across seeds (land exactly 70%, hills 18.2%, mountains 5.6%)
• Verified in a browser against this backend: terrain renders, capital lands on coastal grassland beside a river