fix: emit absolute -fprebuilt-module-path so clangd resolves modules

[Sunrisepeak]

Problem

mcpp emits -fprebuilt-module-path=pcm.cache (bare relative) in the compile command. ninja runs with cwd = <projectRoot>/target/<triple>/<fp>/ so the bare relative path resolves to the actual BMI cache and the build succeeds. But the same flag is captured verbatim into compile_commands.json, whose directory field is set to plan.projectRoot.

Per the CDB spec, clangd does cd <directory> before resolving the arguments. So clangd looks for <projectRoot>/pcm.cache — which doesn't exist. The user-visible symptom is import resolution silently failing in the editor:

module 'mcpplibs.cmdline' not found
All checks completed, 2 errors

… even though mcpp build itself works perfectly.

Root cause

src/build/flags.cppm:215 before:

prebuilt_module_flag = std::format(" -fprebuilt-module-path={}", traits.bmiDir);
//                                                              ^ bare "pcm.cache" / "gcm.cache"

traits.bmiDir is just "pcm.cache" (Clang) or "gcm.cache" (GCC). All the other -fmodule-file= flags in the same block are already wrapped in escape_path(<absolute_path>). This one was a leftover.

Fix

prebuilt_module_flag = std::format(" -fprebuilt-module-path={}",
    escape_path(plan.outputDir / traits.bmiDir));

Single line. ninja still works (absolute paths are cwd-independent). clangd now finds the BMI cache.

Test

New tests/e2e/47_cdb_prebuilt_module_path_abs.sh:

• mcpp new app && mcpp build
• Parse compile_commands.json, extract every -fprebuilt-module-path=X
• Assert each X:
  • is absolute (starts with / on POSIX or <drive>: on Windows)
  • ends in /pcm.cache or /gcm.cache
  • points to an existing directory
• Pass-through when the flag isn't emitted (GCC -fmodules-only flow)

Test plan

• ci-linux (incl. new e2e 47, plus existing 02/03/multi-module tests)
• ci-macos
• ci-windows

Risk

Minimal. The flag is functionally identical (absolute path resolves the same as the cwd-relative path did, since ninja's cwd is exactly <outputDir>). The change only affects how the flag looks in the captured command string.