feat(marking): minimum-viable in-app rubric editor

[hyperpolymath]

Stacked PR (8 of 8 follow-ups). Base = `feature/composer-tutor-voice` (PR #58). Merge order: #51 -> #52 -> #53 -> #54 -> #55 -> #56 -> #57 -> #58 -> this.

Summary

Adds a `/rubric-editor` route where tutors can edit any rubric pack (built-in or disk) as raw `.exs` source, save it to `priv/rubrics/`, and have the composer pick up the new pack on the very next interaction.

Approach: minimum viable

• No structured form. The pack is shown as its inspected `.exs` source.
• Edit the source, change the filename, click Save & reload.
• Validation leans on the runtime-load layer from PR feat(marking): runtime-loaded rubric packs from priv/rubrics/ #56: a broken save is reported via flash but does not remove the working rubrics.
• Revert reloads the on-disk / built-in version.
• Delete on disk removes the `.exs` file; the built-in (if any) is used as the fallback.

New modules

• `EtmaHandler.Marking.RubricSerialiser`
  • `to_source/1` — `inspect` + SPDX header
  • `write_to_disk/3` — writes `/.exs` (appends `.exs` if missing)
  • `delete_from_disk/2` — removes the file
• `EtmaHandlerWeb.RubricEditorLive` — the LiveView itself

Composer integration

• "Edit rubrics →" link next to the existing "Reload rubrics" button so tutors discover the editor from the composer.

Tests

• 68 total, 8 new — Serialiser only; the LiveView itself needs `Phoenix.LiveViewTest` which isn't available in this sandbox
• `to_source` round-trips through `Code.eval_string` to the same struct
• `write_to_disk` writes `/.exs`, appends `.exs` when missing
• `write_to_disk` returns `:no_rubric_dir` when nothing is resolvable
• Written file is loadable by `RubricRegistry.reload` (round-trip)
• `delete_from_disk` removes the file / returns `:enoent` for missing files

Out of scope (future follow-ups)

• Structured form (add/remove components, indicators, questions via UI rather than raw source editing)
• Validation preview before save (currently relies on registry skip)
• Conflict detection when overwriting a disk file that shadows a built-in (just works — disk wins)

Test plan

• 68/68 via standalone elixirc + ExUnit
• LiveView parses cleanly
• Browser: composer -> click "Edit rubrics →" -> editor opens with default rubric source
• Edit the title field in the source, save -> reload picks it up, composer dropdown shows updated title
• Save broken source -> flash shows skip reason, composer still works
• Delete on disk -> falls back to built-in, composer dropdown reflects the rollback

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