Docs: add pre-feature polish train roadmap

[ael-dev3]

Summary

Add the pre-feature polish train roadmap to the main Warpkeep repository as a planning-only PR. It records the seven-PR serial sequence, architecture boundaries, reviewability limits, and release-safety guardrails before broader gameplay work begins.

Base / head

• Base: main @ 582441671f7ad41158ef7117d28985608d3fa96a (Alpha 0.3.13 — The Living Lowlands)
• Head: docs/pre-feature-polish-roadmap @ e4030cb2a3f72b60f79e8cece3ea7cb563217a6b
• Repository: ael-dev3/Warpkeep

Changes

• Add docs/planning/warpkeep-pre-feature-polish-roadmap.md with the seven-step polish train:
  renderer recovery, generic worker authority, worker command-center UX, selectable Water, layered Water/foam/fog, lush Lowlands palette, and bounded forest ecology.
• Document the serial dependency and shared-surface conflict model.
• Record reviewability limits and the distinction between source readiness and production deployment/activation.
• Preserve auth, append-only schema, generated-binding, privacy, browser non-authority, accessibility, reduced-motion, mobile, and no-production-operation guardrails.
• Link the planning document from docs/design/roadmap.md.

PR #79 remains untouched. Open renderer-recovery PRs #80 and #82 are referenced as the current PR 1 lane; no renderer diff is duplicated here.

Verification

• git diff --check
• verify-license-policy.mjs
• verify-file-sizes.mjs
• Full Vitest suite: 196 files, 1,749 tests passed
• TypeScript project build
• Production Vite build

Scope boundary

Documentation only. No runtime code, backend, SpacetimeDB schema or data, authentication, assets, DNS, Pages, deployment, publication, seed, activation, or merge changes are included.