Add Cloud Agent dev environment + mod validation tooling

[IAALAI]

Summary

Sets up a reproducible, repo-managed development environment for the ttl Victoria 3 content mod. The mod has no traditional build system — "development" means editing Paradox script (.txt), localization (.yml), and image assets. The relevant "application" for a mod developer is a validator, so this environment installs the canonical one and adds an offline validation harness.

Because .cursor/environment.json is committed, this becomes the highest-precedence environment source: merging this PR activates the environment for future Cloud Agents (no dashboard Save needed).

Changes

• .cursor/environment.json — repo-managed Cloud Agent environment; runs the install script on setup.
• .cursor/install.sh — idempotent bootstrap that installs vic3-tiger v1.19.0 (the canonical Victoria 3 mod validator) to /usr/local/bin and makes the validation scripts executable. Retries downloads with exponential backoff.
• scripts/validate_mod.py — dependency-free offline well-formedness checker:
  • Paradox script { } brace balance and quote-termination (comment- and string-aware).
  • Localization .yml UTF-8 BOM, l_<language>: header, and KEY:<n> "value" entry format.
  • .metadata/metadata.json JSON validity.
  • .png asset signature integrity.
  • Exits non-zero on error (usable as a CI/pre-commit check).
• scripts/validate.sh — wrapper that runs the offline checks and, when a Victoria 3 install is available via VIC3_DIR, the full vic3-tiger semantic validation.

Why an offline harness

vic3-tiger performs full cross-reference validation but requires the copyrighted Victoria 3 game files, which cannot be shipped in a Cloud Agent VM. The offline harness runs the meaningful checks that do not need the base game, so the environment has a working end-to-end validation flow out of the box. Full semantic validation is a single command away for anyone with the game installed.

Validation

• Install script runs cleanly and is idempotent (second run is a no-op).
• scripts/validate.sh validates all 24 mod files: 0 errors, 0 warnings.
• Verified the checker catches real errors on a deliberately broken fixture (unbalanced brace, missing BOM, unterminated quote, invalid JSON) and exits non-zero.
• Environment build bld-20260806-ae0f6634-b787-4b9e-830e-e1353496187d SUCCEEDED (fresh git checkout + install).
• Booted a fresh Cloud Agent from that build and re-ran all checks: vic3-tiger 1.19.0 and python3 on PATH, setup files present, harness clean (24 files, 0 errors), and broken-fixture detection confirmed.