Add Tom's LQ permanent income trilogy of lectures (#943)

* Add Tom's LQ permanent income trilogy, replacing lq_consumption_smoothing

Splits the single lecture 'LQ Consumption Smoothing: Incomplete Markets,
Complete Markets, and Robust Control' into three lectures:

- lq_permanent_income: the LQ permanent income model
- lq_bewley_complete_markets: cross-section behavior; Bewley and
  complete-markets economies
- lq_robust_smoothing: consumption smoothing under model distrust

Lecture files are taken verbatim from Tom's reference copies, which are
byte-identical to branch pr-931. quant-econ.bib needs no update (the
branch bib is already identical to main).

Supersedes PR #942 (branch pr-931), which conflicted with main and
carried stale pre-review versions of kalman_filter_var.md and
robust_permanent_income.md from before the #931 merge.

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

* Fix duplicate equation labels and stationary-distribution wording

Address Copilot review and the CI build failure on #943:

- Rename eq:pi-endowment -> eq:rs-endowment and eq:pi-crep -> eq:rs-crep
  in lq_robust_smoothing.md; both labels were also defined in
  lq_bewley_complete_markets.md, and the duplicate-label warnings failed
  the jupyter-book build (warnings are errors).
- Reword the Bewley initial-condition passage in
  lq_bewley_complete_markets.md: the unit-root permanent component has
  no stationary distribution, so describe the actual simulation draws
  (z1 ~ N(0,1); transitory z2 from its stationary distribution).

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

---------

Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

Author: mmcky

lectures/_toc.yml

@@ -113,7 +113,9 @@ parts:
   - file: robust_permanent_income
   - file: theil_1
   - file: theil_2
-  - file: lq_consumption_smoothing 
+  - file: lq_permanent_income
+  - file: lq_bewley_complete_markets
+  - file: lq_robust_smoothing
   - file: lq_inventories
 - caption: Optimal Growth
   numbered: true