build: per-sanitizer build dirs to avoid reconfigure churn

[battlesnake]

Problem

All sanitizer flavors write to the same platform build dir (packages/<pkg>/linux, windows, …). self.sanitize isn't part of the build path, so switching between asan / tsan / ubsan / none reuses one dir with a different sanitizer than it was configured for. run_config() then sees the sanitizer marker changed and forces a full CMake reconfigure every switch — slow, and occasionally a stale-mix build failure.

Fix

Suffix the build dir per sanitizer, reusing the existing CLI short tokens:

--sanitize	dir
(none)	linux (unchanged)
address	linux-asan
thread	linux-tsan
undefined	linux-ubsan
leak	linux-lsan

(and the same for every platform: windows-asan, android-tsan, …)

One build_dir_suffix() helper, applied in the two places that pick the per-platform name: platform_build_dir_name() and dependency_chain._save_mama_cmake's get_build_dir_defines (kept in sync per the existing WARNING comment, so dependency mama.cmake include paths match the suffixed dep dirs). No sanitizer ⇒ no suffix ⇒ existing caches stay valid. Each flavor now keeps its own dir + CMakeCache.txt, so switching between already-built flavors no longer reconfigures.

Verification

Added tests/test_build_dir/ asserting the per-sanitizer names and that no-sanitizer is unchanged. Full existing suite (85 tests) green.

Ran the patched mama against a throwaway CMake project: build asan then build tsan create separate linux-asan / linux-tsan dirs with the right enabled_sanitizers; re-running each already-built flavor skips CMake configure (no churn); plain build still uses linux.

Back-compat note

First build of each sanitizer flavor after upgrading mama configures once into its new suffixed dir (the old shared linux dir keeps the no-sanitizer build). Expected, one-time.

[battlesnake]

Superseded by #23 — moved off the fork to a branch on this repo, and adds coverage build-dir separation.