feat(Combinatorics/Quiver/Schreier): word evaluation and reachability

[ZRTMRH]

Adds word evaluation and reachability results to the Schreier graph API.

• SchreierGraph.evalWord : evaluates a word List (S × Bool) as an element of the ambient group, where (s, true) contributes ι s and (s, false) contributes (ι s)⁻¹.
• SchreierGraph.evalWord_eq_lift : agreement with FreeGroup.lift.
• SchreierGraph.evalWord_mem_closure : every word evaluates into the subgroup generated by ι.
• SchreierGraph.pathFromWord : a Bool-tagged word yields a path in Symmetrify (SchreierGraph V ι) from x to evalWord ι w • x.
• SchreierGraph.reachable_iff : two vertices are connected by a path in the symmetrification iff some element of the subgroup closure carries one to the other.

Follow-up to #36320.

This PR was written with AI assistance (Claude). The code has been reviewed by the author.

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PR summary 79540a76d4

Import changes exceeding 2%

%	File
+51.19%	Mathlib.Combinatorics.Quiver.Schreier

Import changes for modified files

Dependency changes

File	Base Count	Head Count	Change
Mathlib.Combinatorics.Quiver.Schreier	295	446	+151 (+51.19%)

Import changes for all files

Files	Import difference
Mathlib.Combinatorics.Quiver.Schreier	151

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Declarations diff

+ evalWord
+ evalWord_append
+ evalWord_cons_false
+ evalWord_cons_true
+ evalWord_eq_lift
+ evalWord_mem_closure
+ evalWord_nil
+ exists_mem_closure_of_path
+ nonempty_path_of_mem_closure
+ pathFromWord
+ reachable_iff

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.

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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).