refactor(Computability.Encoding): unbundle Γ and remove FinEncoding

[AlexeyMilovanov]

This PR unbundles the alphabet Γ from the Encoding structure and completely removes FinEncoding.

Encoding: The alphabet Γ is now an explicit parameter: structure Encoding (α : Type u) (Γ : Type v).
FinEncoding: Removed. Finiteness is now handled via standard typeclasses (e.g., [Fintype Γ] (e : Encoding α Γ)).
Combinators: Functions like finEncodingPair are simplified to encodingPair, dropping the fin prefix and [Fintype] requirements where no longer needed.
Downstream: Mechanically updated Mathlib.Computability and Mathlib.ModelTheory to pass the explicit Γ and use [Fintype Γ] where FinEncoding was previously required.

[github-actions]

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[github-actions]

PR summary 35e1c7ee9f

Import changes for modified files

No significant changes to the import graph

Import changes for all files

Files	Import difference

---

Declarations diff

+ encodingBoolBool
+ encodingList
+ encodingPair
+ unaryEncodingNat
+--+ encoding
- FinEncoding
- FinEncoding.card_le_aleph0
- finEncodingBoolBool
- finEncodingList
- finEncodingNatBool
- finEncodingNatΓ'
- finEncodingPair
- inhabitedFinEncoding
- instance Γ.fintype {α : Type u} (e : FinEncoding α) : Fintype e.toEncoding.Γ
- unaryFinEncodingNat

You can run this locally as follows

## summary with just the declaration names:
./scripts/pr_summary/declarations_diff.sh <optional_commit>

## more verbose report:
./scripts/pr_summary/declarations_diff.sh long <optional_commit>

The doc-module for scripts/pr_summary/declarations_diff.sh contains some details about this script.

---

No changes to technical debt.

You can run this locally as

./scripts/reporting/technical-debt-metrics.sh pr_summary

• The relative value is the weighted sum of the differences with weight given by the inverse of the current value of the statistic.
• The absolute value is the relative value divided by the total sum of the inverses of the current values (i.e. the weighted average of the differences).

[vihdzp]

Seems reasonable to me.