feat(01-12): wave-5 task-1 — welcome i18n migration (conditional on 01-10) + __VITE_DEV__ define + scripts/README.md

Plan 01-10 (welcome tab) has NOT yet landed at execute-plan time
(verified: ls src/welcome/welcome.html returns absent). Per Wave 5
branch 2B, src/welcome/* file modifications are DEFERRED — when Plan
01-10 lands, its executor will use src/shared/tokens.css directly
(skipping the placeholder welcome-tokens.css step entirely; the
canonical tokens.css is already import-ready from src/shared/).

Unconditional changes in this wave:

1. vite.config.ts gains __VITE_DEV__ define-token (RESEARCH §12 +
   D-09 spirit-satisfaction). Defaults to false; activates iff env
   var VITE_DEV=1 is set. Reserved for any future inline smoke-mode
   check. Currently smoke.sh lives entirely outside Vite's input set
   so the gate is a defensive no-op:
     define: { __MOKOSH_UAT__: 'false', __VITE_DEV__: JSON.stringify(...) }

2. vite.test.config.ts inherits __VITE_DEV__ via mergeConfig (the
   test config only overrides __MOKOSH_UAT__: 'true'; __VITE_DEV__
   from base flows through untouched).

3. scripts/README.md (NEW, ~50 lines): documents the smoke-isolation
   invariant — dev-only scripts in scripts/ are NOT bundled by
   `npm run build`; the production dist/ contains zero smoke
   artifacts (verified by RESEARCH §12 grep gate). Provides usage
   example for VITE_DEV env override + cross-references RESEARCH §12
   and brand-decisions-v1.md D-09. Index lists subset-fonts.sh,
   rasterize-icons.sh, and smoke.sh (if present).

Note on Plan 01-10 deferral: when Plan 01-10 executes after this
plan closes, the welcome page src/welcome/welcome.css can either
@import '../shared/tokens.css' directly OR a thin welcome-tokens.css
re-export — both paths are supported by the canonical tokens.css
landed in Wave 1. Plan 01-10's executor must adopt chrome.i18n.getMessage
for any welcome copy strings using the 16-key matrix in _locales/
(welcomeHeroRu + welcomeHeroEn already defined; additional keys
added per Plan 01-10's own artifact list).

Verification:
- vitest baseline 147/147 GREEN (no change from Wave 4 close)
- npm run build clean (no warnings; __VITE_DEV__ propagates through
  define static replacement)
- scripts/README.md exists with the smoke-isolation paragraph

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

vite.config.ts

@@ -38,6 +38,16 @@ export default defineConfig({
   // Reference: https://vite.dev/config/shared-options.html#define
   define: {
     __MOKOSH_UAT__: 'false',
+    // Plan 01-12 Wave 5 (RESEARCH §12 + D-09 spirit-satisfaction):
+    // Defensive token reserved for any future inline smoke-mode check.
+    // Currently the smoke harness lives entirely in `smoke.sh` outside
+    // Vite's input set — verified by `grep -rn 'smoke\|SMOKE\|data:text/html' src/`
+    // returning empty. Activated by setting the env var `VITE_DEV=1`
+    // before invoking `vite` / `vite build`; defaults to `false` so the
+    // gated branch (if any future plan adds one) is statically tree-
+    // shaken out of production. See scripts/README.md for the wider
+    // dev-script isolation invariant.
+    __VITE_DEV__: JSON.stringify(process.env.VITE_DEV === '1'),
   },
   build: {
     // Plan 01-11: bump from default ES2020 to ES2022 so gated top-level