tests: pinned regression suite for #29 + #30 (CI + pre-commit)

[kody-w]

Summary

• Adds a stdlib-only regression test that catches reintroduction of the two project-local-install bugs we just fixed in brainstem UI: drop hardcoded :7071, use same-origin #29 and brainstem: read/write text files as UTF-8 explicitly (Windows fix) #30.
• Wires it into GitHub Actions (runs on every PR touching rapp_brainstem/ and on push to main) and pre-commit (same script as a local hook).
• Each test points at the PR that introduced its guarantee — a future contributor reading a failure can immediately find the bug history.

Why

#29 and #30 were both invisible until someone hit them: the :7071 hardcode looked fine until you ran a project-local install on a different port, and the encoding bug only crashed Windows users on non-UTF-8 system locales. A grep-able test pinned to a specific PR is the cheapest way to keep them dead.

What gets checked

Test	What it asserts	Catches reintroduction of
test_index_html_api_is_same_origin	The const API = … declaration in index.html does not contain a numeric port literal	#29
test_index_html_no_hardcoded_fetch_to_7071	No fetch / XHR / WebSocket / EventSource call site in index.html targets localhost:7071 directly	#29 (defense in depth)
test_no_text_open_without_encoding	AST-walks every .py in rapp_brainstem/ and flags any text-mode open(), Path.open(), Path.read_text(), or Path.write_text() missing encoding=	#30

The AST walker excludes zf.open, urlopen, os.open, subprocess.Popen, webbrowser.open, and binary-mode ("rb"/"wb") opens — these don't have an encoding concept and would otherwise produce false positives.

Manually verified

Reintroduced each regression and confirmed the suite catches them with precise file:line offenders, then reverted and confirmed clean:

$ python3 rapp_brainstem/test_regressions.py
  PASS  index.html declares an `API` constant
  FAIL  index.html `API` constant has no hardcoded port: `API = location.port === '7071' ? '' : ...`
  PASS  index.html has no hardcoded fetch/XHR/WebSocket to :7071
  FAIL  no text-mode open() / read_text / write_text without encoding=utf-8: offenders:
      brainstem.py:771

  2 passed, 2 failed
exit=1

Post-revert: 4 passed, 0 failed.

How to run locally

# direct
python3 rapp_brainstem/test_regressions.py

# via pre-commit (one-time setup: `pip install pre-commit && pre-commit install`)
pre-commit run --all-files

Test plan

• Suite passes on green main (4/4)
• Suite catches reintroduced regressions (2/4 fail with correct offenders)
• YAML for workflow + pre-commit config validates (yaml.safe_load)
• CI run on this PR is green (will verify after push)

Future regressions

When fixing a new bug that "ought to stay fixed," extend rapp_brainstem/test_regressions.py with a test referencing the PR number — the workflow and pre-commit hook pick it up automatically.