Migrate PEx expressions to the shared expression-visitor base

[ankushdesai]

Summary

Phase 2 of the shared expression-visitor refactor. Migrates PEx onto ExpressionGenerator<TContext> (introduced in #951).

PExCodeGenerator now extends ExpressionGenerator<CompilationContext>, and its WriteExpr switch is replaced by the 31 Write*Expr overrides, bodies moved verbatim. PEx's NondetExpr/FairNondetExpr previously shared one switch arm; they're now two overrides routed through a small private WriteRandomBool helper so the emitted code is unchanged.

With PChecker (#951) and PEx both on the base, adding a new IPExpr node forces both backends to handle it or fail to compile.

Stacked on #951 — review/merge that first. This PR's base is claude/shared-expr-visitor; GitHub will retarget it to master once #951 merges.

Verification (local, .NET 8)

• CompilerCore builds clean (only the unrelated pre-existing TransformASTPass.cs:796 CS0162 warning).
• PEx-generated Java for Tutorials 1–6 is byte-for-byte identical to master (diff -rq clean). EspressoMachine fails PEx codegen identically on both old and new — the pre-existing "receive in a loop body is not supported" limitation (TransformASTPass.cs:793), unrelated to this change.

Note on the old default arm

PEx's previous WriteExpr default emitted a /* Skipping expr ... */ comment; the shared base throws instead. Since all three imperative backends handle the identical 31-node set, that arm was dead for real input — confirmed by the byte-identical tutorial output.

Follow-ups

• PR3: WriteStmt base + migrate PChecker/PEx statements.
• PR4: adapt PObserve (signature differs — instance fields vs passed-in writer).

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Generated by Claude Code

[Copilot]

Pull request overview

Phase 2 of the shared expression-visitor refactor: migrates PExCodeGenerator onto the ExpressionGenerator<CompilationContext> base introduced in #951, replacing the single WriteExpr switch with 31 per-node Write*Expr overrides whose bodies are moved verbatim.

Changes:

• PExCodeGenerator now extends ExpressionGenerator<CompilationContext> instead of having its own WriteExpr switch.
• The previously combined NondetExpr/FairNondetExpr switch arm is split into two overrides that delegate to a new private WriteRandomBool helper, preserving emitted output.
• The old default arm that emitted /* Skipping expr ... */ is dropped in favor of the base class's throw (dead in practice).

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