fix: mcpp self config --mirror X hangs on fresh MCPP_HOME

[Sunrisepeak]

Problem

mcpp self config --mirror GLOBAL on a fresh MCPP_HOME hangs because:

1. cmd_self_config → config::load_or_init
2. load_or_init step 5 seeds .xlings.json with the hardcoded default "CN" — regardless of --mirror
3. Step 7 (xlings sandbox bootstrap: xlings self init + ensure_patchelf + ensure_ninja) reads that mirror and downloads via the CN proxy
4. Only after bootstrap finishes does cmd_self_config call xlings::config_set_mirror to update the file — too late, the user's choice never reached the very network roundtrip they were trying to redirect
5. For overseas users / GitHub-runner-style networks the CN mirror stalls → command "hangs"

Fix (~15 lines of real logic)

Plumb the user's mirror choice through to the seed step so the first write of .xlings.json already has the right value, and the bootstrap that immediately follows downloads via the chosen mirror.

• src/config.cppm
  • write_default_xlings_json grows mirror_override parameter (default empty).
  • load_or_init grows initial_mirror parameter (default empty; all 14 existing call sites unchanged).
  • When non-empty, the value is forwarded straight to seed_xlings_json, overriding the "CN" default for that fresh seed.
• src/cli.cppm — cmd_self_config validates --mirror first, then passes it as load_or_init(..., mirror). xlings::config_set_mirror still runs afterwards (no-op on fresh since the seed already has the right value; required for the already-initialized case where the seed is skipped).
• src/xlings.cppm — TODO comment at seed_xlings_json's "CN" default about the long-term direction (flip default to GLOBAL, or auto-detect on first init).

For an already-initialized MCPP_HOME (.xlings.json already exists), the seed is skipped — no behavior change for that path.

Test

New tests/e2e/46_self_config_mirror_no_bootstrap.sh:

• Fresh MCPP_HOME
• mcpp self config --mirror GLOBAL
• Assert .xlings.json mirror is GLOBAL (proves seed override took effect; the OLD code seeded CN and only switched to GLOBAL post-bootstrap)
• Round-trip back to CN via a second --mirror call to exercise the already-initialized path

Existing tests/e2e/38_self_config_mirror.sh still passes — it always calls mcpp self env first (no initial_mirror override), so the "default seed is CN" invariant is preserved.

Diff size

 src/cli.cppm                                    | 32 +++++++----
 src/config.cppm                                 | 23 +++++---
 src/xlings.cppm                                 | 14 +++++
 tests/e2e/46_self_config_mirror_no_bootstrap.sh | 52 ++++++++++++++++++
 4 files changed, 107 insertions(+), 14 deletions(-)

Risk

Minimal. Both new parameters default to empty/unchanged behavior. Only cmd_self_config opts in.

Test plan

• ci-linux (incl. new e2e 46 + existing 38 regression)
• ci-macos
• ci-windows