feat(asm): implement macro system, function calls, and ${param} substitution master
Implement the dfasm macro system as designed in docs/design-plans/2026-02-28-dfasm-macros.md. Grammar changes: - Add macro definition (#name params |> { body }) and invocation syntax - Add function call syntax ($func args |> outputs) with named outputs - Require trailing colon on location directives (disambiguation) - Add scoped references (#macro.&label, $func.&label) - Add ${param} parameter reference syntax for const and edge positions - Add variadic parameters (*args) and repetition blocks ($(body),*) Pipeline: - Lower: CST-to-IR for all new grammar rules, MacroDef/IRMacroCall/ParamRef creation, token pasting pattern detection - Expand: new pass between lower and resolve. Macro expansion with parameter substitution, const expression evaluation, token pasting, variadic repetition. Function call wiring with @ret return markers and per-call-site context slots. - Allocate: per-call-site context slot assignment via CallSite metadata - Codegen: CTX_OVRD (ctx_mode=01) emission on cross-call-site edges Built-in macro library: zero-param self-contained macros (dup, discard, reduce_add) prepended to user source automatically. Error handling: MACRO and CALL error categories with expansion stack threading for precise error locations in nested macros. Tests: 1034 passing (40 new macro/call tests + existing regression suite).