perf(client): main-thread memory harness + drop three map-sized render buffers (-23%) (#4511)

## Summary

Follow-up to #4507, moving the memory-footprint campaign to the **main
thread** (client). Two parts: a headless browser measurement harness,
and a first optimization round that cuts the main-thread live heap on
Giant World Map by **23%** (166 → 128 MB at tick 2000).

## Part 1 — `npm run perf:client-mem`: headless main-thread memory
harness

Drives a real singleplayer game in headless Chromium and measures the
**page's isolate only** (the core sim worker is a separate CDP target):

- Starts its own vite dev server on a private port (default 9017) so it
always measures the current checkout.
- Double-forced-GC checkpoints every `--window` ticks: JS heap,
ArrayBuffer backing-store bytes (`Runtime.getHeapUsage`), DOM nodes,
listeners, ticks/s.
- `--snapshot-at <ticks>` writes V8 heap snapshots, analyzable with the
retainer/summary tools from #4507.
- Spoofs the unmasked WebGL renderer string via an init script so the
software-GL gate (#4324) admits SwiftShader — no game code touched;
rendering still runs software (hence the rAF throttle).
- End-of-run screenshot as a rendering sanity check.

Baseline (Giant World Map, 400 bots, 12,000 ticks): ~176 MB live, of
which ~116 MB is **static per-tile buffers** allocated up front for the
8M-tile map — flat during play, no leaks.

## Part 2 — drop three map-sized render-layer buffer copies

| Buffer | Before | After |
|---|---|---|
| `TrailPass.cpuTrailState` | 15.3 MB copy | **deleted** — dead code;
every upload entry point sets the live reference to TrailManager's array
|
| `RailroadPass.cpuRailroadState` | 15.3 MB across 2 arrays | references
`RailroadCache.railroadState` (stable identity, mutated in place) |
| `RailroadPass.cpuGhostRailState` | ↑ | sparse `Map<ref, value>`;
preview diffs applied as per-texel `texSubImage2D` writes (path-sized
work instead of a full 8 MB texture upload per build-preview mouse move)
|
| `TerrainPass` + `MapRenderer` terrain bytes | 7.6 MB (one buffer, two
retainers) | `terrainSource()` provider — re-bakes (theme change,
context restore) regenerate from the live game map, which already
reflects water-nuke conversions |

Tick-2000 snapshot comparison (giant world, 400 bots): **166.4 → 128.4
MB**.

## Verification

- `tsc --noEmit`, eslint, full test suite (1924 tests) pass.
- 2000-tick headless giant-world game after the change: no GL
pageerrors, end-of-run screenshot renders
terrain/territory/borders/names correctly, sim speed unchanged (~5
ticks/s headless).
- Ghost-rail ops flush before the zoom-fade early-return, so the op
queue can't grow while previewing at low zoom.
- WebGL context restore recreates all passes fresh and the owner
re-uploads state (existing `onContextRestored` path), consistent with
the new reference-based buffers.

Note: heap snapshots in `tests/perf/output/` are gitignored; the numbers
above are from runs recorded in the PR discussion.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

---------

Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

package.json

@@ -16,6 +16,7 @@
     "perf": "npx tsx tests/perf/run-all.ts",
     "perf:game": "npx tsx tests/perf/fullgame/FullGamePerf.ts",
     "perf:client": "npx tsx tests/perf/client/ClientUpdatePerf.ts",
+    "perf:client-mem": "npx tsx tests/perf/client/ClientMemoryPerf.ts",
     "test:coverage": "vitest run --coverage",
     "format": "prettier --ignore-unknown --write .",
     "format:map-generator": "cd map-generator && go fmt .",