feat: implement Phase 1 — manifest, codegen, and ABI modules

Replace scaffold stubs with full Nim code generation pipeline:

- Manifest: [project], [[functions]] (name, params, return-type, pragmas),
  [nim] (backend: c/cpp/js, gc: arc/orc/none, opt-level) with validation
- ABI: NimProc, NimType, NimPragma, NimTemplate, CompilationTarget, GcStrategy
  with Display impls and rendering methods
- Codegen/parser: parse manifest function signatures into typed ABI NimProc
  structs, handling ptr/ref/seq/array types
- Codegen/nim_gen: generate .nim files with proc declarations, pragmas,
  stub bodies, doc comments, and wrapExportc helper template
- Codegen/build_gen: generate nim compilation commands, nim.cfg, build.sh
- 38 tests (28 unit + 10 integration), all passing, zero warnings
- Example manifest in examples/fast-lib/

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: hyperpolymath

Cargo.lock

@@ -0,0 +1,58 @@
+# SPDX-License-Identifier: PMPL-1.0-or-later
+# Example nimiser manifest: fast-lib
+# A minimal math library generated via Nim metaprogramming.
+
+[project]
+name = "fast-lib"
+version = "0.1.0"
+description = "A fast math library compiled from Nim to C"
+author = "Jonathan D.A. Jewell <j.d.a.jewell@open.ac.uk>"
+
+[[functions]]
+name = "fast_add"
+return-type = "cint"
+pragmas = ["exportc", "cdecl"]
+doc = "Add two C integers with zero overhead."
+
+  [[functions.params]]
+  name = "a"
+  type = "cint"
+
+  [[functions.params]]
+  name = "b"
+  type = "cint"
+
+[[functions]]
+name = "fast_multiply"
+return-type = "cdouble"
+pragmas = ["exportc", "cdecl", "noSideEffect"]
+doc = "Multiply two C doubles."
+
+  [[functions.params]]
+  name = "x"
+  type = "cdouble"
+
+  [[functions.params]]
+  name = "y"
+  type = "cdouble"
+
+[[functions]]
+name = "fast_negate"
+return-type = "cint"
+pragmas = ["exportc", "cdecl", "inline", "noSideEffect"]
+doc = "Negate a C integer."
+
+  [[functions.params]]
+  name = "val"
+  type = "cint"
+
+[[functions]]
+name = "fast_init"
+return-type = ""
+pragmas = ["exportc", "cdecl"]
+doc = "Initialise the library (no-op stub)."
+
+[nim]
+backend = "c"
+gc = "arc"
+opt-level = "speed"