refactor: decompose method-calls.ts — extract map/set/urlsearchparams dispatch

[cs01]

Summary

• Extracts ~490 lines of map, set, and URLSearchParams dispatch logic from method-calls.ts into 3 focused sub-modules
• method-calls.ts reduced from 1253 to 763 lines
• New files: map-dispatch.ts (277 lines), set-dispatch.ts (107 lines), urlsearchparams-dispatch.ts (73 lines)
• No behavioral changes — pure refactor with identical codegen output

Test plan

• npm run verify:quick passes (tests + Stage 0 + Stage 1 self-hosting)
• All map/set/urlsearchparams tests pass with both node and native compiler