fix: review fixes + feat: imports, assertions, merge engine, per-package overrides

Review fixes (8 blocking issues):
- Extract ParseError to shared error.hpp (ODR fix)
- Remove dead package.cpp/package.hpp + tomlplusplus dep
- Safe parse_int() helper replacing crash-prone std::stoi
- consume_string() now accepts bare numbers and idents
- line_at() fixed for post-EOF line numbers
- Subcommand validation before file read in CLI
- KwService/KwAssert/KwImport added to consume_ident()
- root partition promoted to first-class BootBlock field

New features:
- Imports: imports = [...] with recursive merge resolution
- Assertions: assert { "msg" : field op value } in both parsers
- Merge engine: resolve_package() with features/config merge + force support
- Per-package overrides: /kappa/system/builds/<name>.kap
- .gitignore: added vcpkg_installed/ and kappa binary

examples/config.kap

@@ -2,6 +2,14 @@
  * Kappa system configuration.
  * Lives at /kappa/system/config.kap
  */
+
+imports = []
+
+assert {
+    "efi partition required for UEFI boot" : boot.efi != ""
+    "root partition must be set"           : boot.root != ""
+}
+
 system {
     hostname = "kappa.local"
     timezone = "America/New_York"