MAINT: tidy up orphaned _static/lecture_specific/optgrowth_fast/ (and related optgrowth-series) files

Background

lectures/_static/lecture_specific/optgrowth_fast/ contains two code snippets (ogm.py, ogm_crra.py)
that were :load:-ed by the old lecture optgrowth_fast.md ("Optimal Growth II: Accelerating the Code
with Numba").

That lecture was deleted in #679 (merged 2025-11-08), which refactored the optimal growth series into
the cake eating series (now the os_* lectures) with inline NamedTuple models. The refactor inlined
or removed the other external code files it used, but this folder was left behind.

Current state

• No lecture references optgrowth_fast/: there are no :load: directives or other references to
  ogm.py / ogm_crra.py anywhere in the repo, and OptimalGrowthModel appears nowhere outside these
  two files. (The only lecture still loading a static .py is markov_perf.)
• Further evidence the files are never executed: ogm_crra.py defines u_prime_inv(c) without self
  while its body references self.γ, which would fail on any call.
• Two other leftovers from the same refactor also appear unreferenced:
  • _static/lecture_specific/optgrowth/ (3ndp.pdf, solution_og_ex2.png)
  • _static/lecture_specific/coleman_policy_iter/ (solve_time_iter.py)
• Two open PRs from the QuantEcon/meta#299 RNG migration touch these files: #962 (ogm.py) and
  #963 (ogm_crra.py). Both are blocked on a review question about changing the constructor
  signature — which becomes moot if the files are removed.

Proposed tidy-up

• Confirm no external consumers (e.g. other lecture repos fetching these files by raw URL)
• Delete lectures/_static/lecture_specific/optgrowth_fast/
• Delete (or confirm still wanted) _static/lecture_specific/optgrowth/ and
  _static/lecture_specific/coleman_policy_iter/
• Close or repurpose #962 and #963 accordingly