Feat: Package-relative source paths and a structured 'why no C' report

[b-re-w]

Closes #112
Closes #128

Features added

• TypedPython: embed a package-relative source path, not the host's absolute path #112: the generated C embeds the module's path relative to the project root (posix), not the host's absolute path. compile_project passes it, and compile_module(project_root=None) defaults to the source's directory, so only the file name is embedded. Compiling the same project from two different absolute roots gives byte-identical C.
• TypedPython: structured per-module 'why no C' report for pypackpack's native level #128: ModuleResult gains kind (compiled, nothing_compiled, not_marked) and skipped_items: list[SkippedItem(name, file, line | None, message)], the shape pypackpack Desktop: ship the stdlib with a packaged app (jar/jpackage), not only via PYTHONHOME for Gradle tasks #60's NotNativeModule consumes. python -m typedpython build prints file:line: name: message per item, and one line for a module not marked for compilation. ModuleResult.skipped is unchanged, so existing callers keep working.

Behaviour change

• The incremental cache key now includes the module's relative path, because the path is part of the C. A moved file with identical content rebuilds. The incremental.py docstring says so.

Design note

• ir.Module.skipped stays dict[str, str], because the interface hash reads its keys. The structure is built in the pipeline: the front end's line N: prefix is parsed off, and verifier skips take the function's first diagnostic line.

Tests added

• 13 tests in test_module_report.py: all 13 failed before the implementation.
• Mutation: embedding the absolute path again makes 3 tests fail (two-roots identity, compile_module embedding, moved file).

Verification (local, macOS arm64): full TypedPython suite, 947 passed, 3 skipped, EXIT=0. The later merge of develop touched no file under python-multiplatform-ksp.