fix: restore operator overloads, fix casts, imports, and compound assignment

[danfma]

Summary

Pack of fixes discovered while adding the SampleOperatorOverloading project:

• Operator auto-naming: derive names from C# token (+ → add, * → multiply) when [Name] is absent — operators were silently skipped before
• Overload dispatcher for operators: same-token overloads (e.g., Money * bigint vs Money * Money) now generate runtime dispatchers with type checks
• Numeric cast transpilation: explicit casts between bigint/Decimal/number now emit correct code (new Decimal(x.toString()), BigInt(x.toFixed(0)), etc.)
• Math.Round/Floor/Ceiling with decimal: rewritten to Decimal.js instance methods when argument is System.Decimal
• Import merge: multiple imports from the same path consolidated into one line
• Top-level statement fixes: const → let for mutated locals, relative imports instead of barrel alias, compound assignment (+=) lowered to $add() call
• BigInt literals: numeric literals implicitly converted to BigInteger emit with n suffix
• Add SampleOperatorOverloading project with Money record struct and 13 TS tests

Test plan

• 357 .NET tests pass
• 455 JS tests pass (18 + 65 + 19 + 13 + existing)
• Zero TS compilation errors in SampleOperatorOverloading
• All other samples build and test cleanly

Closes #53
Closes #54

[Copilot]

Pull request overview

This PR fixes several TypeScript transpilation edge cases discovered while adding the new SampleOperatorOverloading sample, primarily around operator overloading, numeric casts, import generation/merging, and top-level statement handling.

Changes:

• Add operator auto-naming and overload dispatch for user-defined operators, plus compound-assignment lowering (+= → $add()).
• Fix numeric literal/cast emission for BigInteger/decimal/bigint interop and rewrite Math.Round/Floor/Ceiling for decimal.
• Consolidate duplicate imports, adjust top-level statement mutation detection, and add the SampleOperatorOverloading sample + TS tests.

Reviewed changes

Copilot reviewed 34 out of 34 changed files in this pull request and generated 8 comments.

Show a summary per file

File	Description
tests/Metano.Tests/TypeMappingTranspileTests.cs	Updates UUID import assertions to accommodate merged imports.
tests/Metano.Tests/OperatorTranspileTests.cs	Adds coverage for operator auto-derived names and [Name] overrides.
src/Metano.Compiler.TypeScript/Transformation/StatementHandler.cs	Fixes mutation detection scope for top-level statements.
src/Metano.Compiler.TypeScript/Transformation/RecordClassTransformer.cs	Adds operator token→name mapping and operator overload dispatcher generation.
src/Metano.Compiler.TypeScript/Transformation/OperatorHandler.cs	Lowers user-defined compound assignments to $opName calls.
src/Metano.Compiler.TypeScript/Transformation/LiteralHandler.cs	Emits n-suffixed bigint literals when targeting BigInteger.
src/Metano.Compiler.TypeScript/Transformation/InvocationHandler.cs	Rewrites System.Math calls for decimal arguments to decimal.js instance methods.
src/Metano.Compiler.TypeScript/Transformation/ImportCollector.cs	Merges multiple imports from the same module path into one statement.
src/Metano.Compiler.TypeScript/Transformation/ExpressionTransformer.cs	Implements explicit cast lowering for decimal/BigInteger conversions.
src/Metano.Compiler.TypeScript/PackageJsonWriter.cs	Adjusts # import alias generation based on presence of root barrel.
spec/08-feature-support-matrix.md	Updates roadmap/status notes for packaging and cross-package behavior.
spec/04-functional-requirements.md	Extends FR-030 and adds FR-046/FR-047 requirements.
samples/SampleOperatorOverloading/SampleOperatorOverloading.csproj	Adds new sample project wired to transpiler output.
samples/SampleOperatorOverloading/Program.cs	Adds top-level-statement sample exercising imports and compound assignment.
samples/SampleOperatorOverloading/NoSameMoneyCurrencyException.cs	Adds sample exception type for currency mismatch.
samples/SampleOperatorOverloading/Money.cs	Adds Money type with overloaded operators and decimal/BigInteger conversions.
samples/SampleOperatorOverloading/Currency.cs	Adds Currency enum for sample.
js/sample-todo/src/todo-list.ts	Consolidates metano-runtime imports into a single line.
js/sample-todo-service/src/todos.ts	Consolidates metano-runtime imports into a single line.
js/sample-todo-service/package.json	Adds "#" import mapping to root barrel.
js/sample-operator-overloading/tsconfig.json	Adds TS build config for the new operator-overloading JS sample.
js/sample-operator-overloading/test/money.test.ts	Adds Bun tests covering generated operator helpers and dispatch.
js/sample-operator-overloading/src/program.ts	Adds generated program output verifying top-level statement fixes.
js/sample-operator-overloading/src/no-same-money-currency-exception.ts	Adds generated exception class output.
js/sample-operator-overloading/src/money.ts	Adds generated Money class with operator helpers and dispatchers.
js/sample-operator-overloading/src/index.ts	Adds generated barrel file for the sample.
js/sample-operator-overloading/src/currency.ts	Adds generated Currency enum output.
js/sample-operator-overloading/package.json	Adds JS package manifest for the new sample.
js/sample-issue-tracker/src/shared-kernel/user-id.ts	Consolidates metano-runtime imports into a single line.
js/sample-issue-tracker/src/planning/domain/sprint.ts	Consolidates metano-runtime imports into a single line.
js/sample-issue-tracker/src/issues/domain/issue-id.ts	Consolidates metano-runtime imports into a single line.
js/sample-issue-tracker/src/issues/application/issue-queries.ts	Consolidates metano-runtime imports into a single line (incl. type import).
Metano.slnx	Adds SampleOperatorOverloading project to the solution.

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