add IR builder methods to BaseGenerator and IGeneratorContext

[cs01]

Summary

• Add 7 IR builder methods to BaseGenerator, IGeneratorContext, and MockGeneratorContext (three-way sync): emitCall, emitCallVoid, emitLoad, emitStore, emitGep, emitIcmp, emitBitcast
• Each method auto-allocates a temp register, emits the LLVM IR instruction, tracks the result type via setVariableType(), and returns the register name
• Pilot migration: convert DateGenerator.generateNow() in date.ts to use the new builders as proof of concept

This is Shard C of the compiler rearchitecture plan. All files are disjoint from Shards A/B/D.

Files changed

• src/codegen/infrastructure/base-generator.ts — builder method implementations
• src/codegen/infrastructure/generator-context.ts — interface declarations + mock implementations
• src/codegen/stdlib/date.ts — pilot migration (5 of 10 nextTemp+emit pairs replaced with single-line builder calls)

Test plan

• npm run build passes clean
• npm test — 324 passing, 18 failing (all pre-existing try-catch failures, identical to clean main)
• npm run verify:quick — self-hosting stages pass, same pre-existing failures only