fix(lean): make lake build actually compile and pass + gitignore build artifacts

[Kairose-master]

Summary

Same CI investigation as #22 (feature/gemini-proposal) and #23 (main), applied to experiment. This branch's Core/EpsFunctor.lean and Core.lean are yet another distinct revision of the design (Core.lean here imports Core.RawPrefunctor/EpsFunctor/Tactics directly, unlike main's single-file version or feature/gemini-proposal's CatPrism.lean aggregator), so the fixes are specific to this variant rather than a copy-paste of the earlier ones.

lake build (CI's proof-check step) was failing against the pinned mathlib (v4.21.0-rc2):

• EpsFunctor.comp_ok: composition order was backwards (g ≫ f when f : A ⟶ B, g : B ⟶ C₁ don't compose that way) — swapped to f ≫ g.
• EpsFunctor: dropped an incorrect manual Type (u+1) universe ascription, letting Lean infer the correct universe.
• EpsFunctor.fromStrict: took Decidable (d f f = 0), which only proves the equality is decidable, not that it holds — replaced with an actual reflexivity hypothesis (hd : ∀ {A B} (f : A ⟶ B), d f f = 0). Also fixed a broken anonymous-constructor refine { ..., ?_, ?_ } (mixing named and unnamed fields is invalid syntax) by switching to a where clause, and made C/D explicit via @EpsFunctor C D _ _ d 0 since they were left as stuck metavariables during typeclass search.
• Core.lean: open Core.EpsFunctor referenced a namespace that doesn't exist — Core/EpsFunctor.lean declares namespace EpsFunctor, not namespace Core.EpsFunctor — fixed to open EpsFunctor.

Also adds .gitignore for build artifacts (parser/target/, core/lean/.lake/, core/lean/AutoGen/) — the repo had none, so reproducing CI locally left them as untracked output.

Test plan

• Ran lake update && lake build locally on this branch — clean build (one harmless unused-variable warning only).

Known remaining gap (out of scope here)

CatPrism_examples.lean (not a CI default target) still references names that don't match what the .cat → .lean generator emits — same gap noted on the other two branches. Left for a follow-up.

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