docs(@projects): 📝 track regression in p1-29c follow-up doc

Co-Authored-By: Lilith Autocommit <noreply@atlilith.com>

.project/objectives/p1-29c-followup-empty-params-json-regression.md

@@ -0,0 +1,50 @@
+---
+id: p1-29c-followup-empty-params-json-regression
+title: "GdEconomy::process_turn fails — `_build_params_json` produces empty string for autoplay seeds"
+priority: p1
+status: stub
+scope: game1
+category: bug
+owner: shipwright
+created: 2026-05-14
+updated_at: 2026-05-14
+blocked_by: []
+follow_ups: [p1-29c]
+---
+
+## Context
+
+`p1-29c` autoplay batch ran on apricot 2026-05-14 (`autoplay_batch_p1_29c-1778813509`, DONE 20:08). All 12 seeds **FAILED** the E2E gate with hundreds of repeated errors:
+
+```
+ERROR: GdEconomy::process_turn params_json parse: EOF while parsing a value at line 1 column 0
+```
+
+The cities_json + units_json args parsed OK (no preceding parse errors logged) — only `params_json` is empty.
+
+Caller: `src/game/engine/src/modules/empire/economy.gd:43`
+```gdscript
+var params_json: String = _build_params_json(player)
+var result: Dictionary = gd_economy.process_turn(cities_json, units_json, params_json)
+```
+
+`_build_params_json` ends with `JSON.stringify({"yield_mult": GameState.get_effective_yield_mult(player, "gold"), ...})`. Empty string output from `JSON.stringify` happens on degenerate inputs (NaN, +/-Inf, cyclic refs). Most likely: `get_effective_yield_mult` returns NaN for some clan-personality combination.
+
+## Acceptance
+
+- ☐ Reproduce: pick one failing seed, run with verbose logging, capture the `_build_params_json` inputs that trigger the empty stringify.
+- ☐ Identify the input that breaks stringify (likely `yield_mult` NaN).
+- ☐ Fix at source: either guard `get_effective_yield_mult` to clamp NaN→1.0, or guard `_build_params_json` to substitute a sentinel.
+- ☐ Re-run `autoplay-batch.sh 10 300` on apricot; E2E gate ≥10/12 PASS (allow 2 unrelated environmental failures).
+- ☐ Then unblock `p1-29c` final acceptance bullet (tier_peak ≥2 in ≥7/10 alive-aware seeds).
+
+## References
+
+- Batch log: `apricot:~/.local/var/p1_29c/run.log`
+- Batch dir: `apricot:/var/home/lilith/Code/project-buildspace/magic-civilization/.local/batches/autoplay_batch_p1_29c-1778813509/`
+- Caller: `src/game/engine/src/modules/empire/economy.gd:_build_params_json`
+- Sink: `src/simulator/api-gdext/src/lib.rs:6221` (the godot_error site).
+
+## Why this blocks p1-29c closure
+
+The batch infrastructure must produce clean games before the tier-peak gate (`p1-29c` bullet 1) can be meaningfully measured. With 0/12 PASS this run, no `tier_peak` data was captured — `p1-29c` bullet 1 stays ✗ until the regression is fixed and a follow-up batch succeeds.