Rename pre-refactor lecture files to current source slugs; drop 3 superseded

Aligns the 8 orphaned files with the source reorganisation in
lecture-python.myst (commits b7c6464 and b27f1eb, Nov 2025):

Renamed (translation preserved, content resync follows in the Track B wave):
- cake_eating_problem.md -> os.md
- cake_eating_numerical.md -> os_numerical.md
- optgrowth.md -> os_stochastic.md
- ifp.md -> ifp_egm.md
- mccall_correlated.md -> mccall_persist_trans.md

Removed (deleted upstream in the cake-eating refactor; successors
os_time_iter/os_egm/os_egm_jax are new untranslated lectures, Phase 2):
- optgrowth_fast.md
- coleman_policy_iter.md
- egm_policy_iter.md

Part of Phase 0 Track B, QuantEcon/lecture-python.myst#947.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Author: mmcky

lectures/_toc.yml

@@ -66,7 +66,7 @@ parts:
   - file: mccall_model
   - file: mccall_model_with_separation
   - file: mccall_fitted_vfi
-  - file: mccall_correlated
+  - file: mccall_persist_trans
   - file: career
   - file: jv
   - file: mccall_q
@@ -79,13 +79,10 @@ parts:
   - file: cass_fiscal
   - file: cass_fiscal_2
   - file: ak2
-  - file: cake_eating_problem
-  - file: cake_eating_numerical
-  - file: optgrowth
-  - file: optgrowth_fast
-  - file: coleman_policy_iter
-  - file: egm_policy_iter
-  - file: ifp
+  - file: os
+  - file: os_numerical
+  - file: os_stochastic
+  - file: ifp_egm
   - file: ifp_advanced
 - caption: LQ控制
   numbered: true