[A3] Publish scaling laws assignment

[GQH123]

Summary

• Publish the student-facing A3 scaling-laws assignment handout.
• Define the exploratory API workflow and centralized final evaluation phase.
• Specify the required experiment evidence, fitting report, point prediction,
  and 80% central prediction interval.
• Add the grader supplement covering the 60/20/20 scoring model, prediction
  penalties, methodology rubric, integrity checks, and exceptional cases.

Scope

This is a documentation-only change under `assignments/A3/`.

The control-plane API, compute-side training backend, data pipeline, and
deployment packages remain in the separate infrastructure project. Adding
that project as a source-code submodule is intentionally deferred until the
implementation has been fully archived.

Validation

• `git diff --check`
• Markdown fence and relative-link validation
• Syntax parsing of embedded Python examples
• Existing maintained submission-tool tests: `13 passed`
• Large-file check: no files over 100 MB

Known Repository-Wide Checks

The complete repository validator still reports pre-existing student-record
issues, including missing Feishu URLs and placeholder values outside A3.

Default repository-wide pytest collection also requires `torch` for an
existing student smoke test. Neither issue is introduced or modified by this
documentation-only change.