feat: compile .c sources via a separate C rule

[Sunrisepeak]

Summary

Adds a parallel C-language compile path so mcpp can build packages with
.c sources (and depend on real-world C libraries like mbedtls). Previously
every non-.cppm source went through one cxx_object rule with
-std=c++23 -fmodules; cc1plus rejects mainstream C constructs (implicit
void* conversion from malloc/calloc, restrict, ...), so the only
projects that built were ones whose .c files happened to be C++-clean.

What changed

• BuildConfig (src/manifest.cppm) gains cflags / cxxflags / cStandard.
  Both the [build] TOML section and the mcpp = {} xpkg segment
  (synthesize_from_xpkg_lua) parse them — index entries can carry
  per-package flags.
• src/build/ninja_backend.cppm: when the plan contains any .c source,
  derive a sibling C compiler from the resolved C++ binary
  (g++ → gcc, clang++ → clang, c++ → cc, prefix preserved for
  cross toolchains like x86_64-linux-musl-gcc) and emit cc / cflags /
  c_object alongside the existing cxx_* set. cflags defaults to
  -std=c11 plus the shared opt / pic / sysroot / -B / include baseline
  and the user tail.
• Compile-edge dispatch picks cxx_module for .cppm, c_object for
  .c, cxx_object otherwise. Dyndep phase-1 skips .c (no P1689
  scan / -fmodules); the static-deps fallback skips BMI implicits for
  the same reason.
• src/modgraph/scanner.cppm short-circuits .c with an empty
  SourceUnit so benign C identifiers (import_foo, module_t) can't
  be misparsed as module statements.

Verification

• Unit: two new cases in tests/unit/test_manifest.cpp cover both parse
  paths ([build] and mcpp = {} segment).
• E2E: new tests/e2e/26_c_language_support.sh scaffolds a mixed
  .c + modular-.cpp project, asserts build.ninja has c_object /
  cc / cflags, exercises user-supplied flag forwarding, and runs
  the resulting binary end-to-end.
• Real library: built mbedtls 3.6.1 as a path dependency. All 108
  library/*.c files compile via the new c_object rule and link
  into a binary that prints the FIPS 180-4 sha256("abc") test
  vector and an AES-128-ECB sample correctly.
• Full local e2e suite: 30 passed, 2 failed. The two failures
  (28_target_static.sh, 30_pack_modes.sh) are pre-existing — both
  reproduce on the v0.0.1 release binary unchanged.

Test plan

• mcpp build (worktree, builds mcpp itself)
• mcpp test — all 9 unit binaries pass
• tests/e2e/26_c_language_support.sh (new)
• Mixed-language smoke project: restrict + implicit void* + import std
• mbedtls 3.6.1 path dep: 108 .c → linked binary → FIPS test vector
• CI green