bug: Ninja object paths collide for same-basename sources with different extensions #149

What happened?

The Ninja backend derives object paths by replacing each source suffix with .o. Within one target, source files that share a basename but use different extensions therefore map to the same output.

For example, src/start.c and src/start.S both generate _build/obj/app/src/start.o. Ninja rejects the generated manifest because multiple build rules declare the same output, so valid mixed C and assembly targets cannot build.

Expected behavior

Object paths should retain enough of the complete source filename to remain unique while preserving target namespacing:

• src/start.c → _build/obj/app/src/start.c.o
• src/start.S → _build/obj/app/src/start.S.o

The compilation database should use the same object paths as the Ninja manifest.

Reproduction steps

1. Define one executable target containing src/start.c and src/start.S.
2. Generate the Ninja build manifest.
3. Inspect the object outputs or run ninja -n.
4. Observe that both compile rules declare _build/obj/app/src/start.o, causing Ninja to reject the manifest with a duplicate-output error.

Version or commit

Upstream master before PR #133; reproduced at the parent of commit 5cc570d

Environment

Platform-independent Ninja manifest generation. Reproduced in the Python Ninja backend tests on Windows; the generated manifest was also checked with a real Ninja dry run.

Logs or diagnostics

Submission checks

• I searched existing issues for a duplicate.
• This report contains no security-sensitive details.