feat(marking): runtime-loaded rubric packs from priv/rubrics/

[hyperpolymath]

Stacked PR (5 of 5 follow-ups). Base = `feature/composer-borderline-detection` (PR #55). Merge order: #51 -> #52 -> #53 -> #54 -> #55 -> this.

Summary

Tutors can add a new exam rubric by dropping an `.exs` file into `priv/rubrics/` and clicking "Reload rubrics" in the composer UI — no recompile, no restart. Built-in rubrics stay as compile-time fallbacks; disk-discovered packs can shadow them on `id` collision (so `priv/rubrics/ou_3_part.exs` overrides the built-in OU pack without a code change).

What ships

• `RubricRegistry` refactor
  • `@builtin_packs` (`ou_3_part`, `single_essay`) remain compile-time fallbacks
  • `all/0` returns disk packs ++ built-ins, with disk shadowing on `id` collision
  • First read triggers a lazy disk scan, cached in `:persistent_term`
  • `reload/0` re-scans and returns `%{loaded: N, skipped: [...]}`
  • `rubric_dir/0` configurable via `Application.get_env(:etma_handler, :rubric_dir)`, falling back to `:code.priv_dir(:etma_handler) <> "/rubrics"` (`nil` when the app isn't loaded — keeps standalone module tests viable)
• Robustness: files that fail to evaluate / return a non-RubricPack / have an empty `id` are skipped with a reason — a single broken file does not take the whole registry down.
• LiveView: "Reload rubrics" button next to the rubric selector. Result surfaced via `put_flash` with loaded count and per-file skip reasons.
• `priv/rubrics/`:
  • `README.adoc` documents the file shape and the shadowing rule
  • `example_extended.exs` is a working three-component sample (method / analysis / conclusion) demonstrating the loading pipeline

Authoring a new rubric

Drop a file like:

```elixir
alias EtmaHandler.Marking.RubricPack

%RubricPack{
id: "my_course_t1",
title: "My course, TMA 1",
components: [%{id: "essay", code: "a", title: "Essay", short: "essay", good: [{"...", "..."}], improve: [{"...", "..."}]}],
default_questions: [%{"id" => "q1", "label" => "Essay", "mark" => "", "max" => "100", "component_id" => "essay"}]
}
```

into `priv/rubrics/`, click Reload rubrics, pick the new rubric in the selector. Done.

Tests

• 50 total, 5 new runtime cases (`async: false`, isolated via per-test tmp dir + `:persistent_term` erase on exit)
  • `reload/0` picks up a freshly written `.exs`
  • Disk pack with the same id shadows the built-in
  • Malformed files are skipped with a reason (syntax error, wrong shape, missing id)
  • Built-ins remain available when the rubric directory is empty
  • `reload/0` with `nil` rubric_dir is a no-op (not a crash)

Test plan

• 50/50 module + registry + runtime tests pass via standalone elixirc + ExUnit
• LiveView syntax-checks
• In browser: `mix phx.server` -> /exam-feedback-composer -> "Example: extended essay…" shows in the selector
• Copy `example_extended.exs` to e.g. `my_test.exs`, edit the `id` and a label, hit Reload -> new rubric appears
• Write a syntactically broken `.exs` file in `priv/rubrics/` and Reload -> flash shows the skip reason; built-ins still work

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