fix(pwndbg): reject core mapping paths that escape sysroot

Interpret source-bound ELF64 NT_FILE descriptors once before debugger
launch. Bound counts and pathname coverage by the descriptor, retain exact
path bytes, and refuse relative names or dot segments rather than letting
GDB resolve them against the host. Pin GDB's pathname semantics to Unix.

Preserve the decoded partial observation on refusal, basic-only recovery,
and pre-admission cancellation. Share an exact mapping fixture and replace
the old fixture boolean modes with explicit report producers.

Validation: final series has 33 passing Vitest cases plus five Python
producer controls; 38 Linux x64 provider cases are explicitly skipped on
macOS. Typecheck/lint, docs checks and test discovery pass. Real GDB16.3
and strace reproduce the relative-path escape using only an owned fixture;
compiled admission admits a real absolute-path core and refuses that
relative-path core. Full native pwndbg / Linux x64 remain unverified here.
Task token, CPU and peak-family-memory costs are unmeasured; no speed claim.

Fixes #4942

bridge/pwndbg/core_context.py

@@ -20,7 +20,7 @@ def inspect(request):
     root = Path(os.environ["REA_CORE_CONTEXT_REQUEST"]).parent
     empty = root / "no-libraries"
     empty.mkdir(mode=0o700)
-    for command in ["set auto-load off", "set debuginfod enabled off", "set auto-solib-add off", "set history save off", "set pagination off",
+    for command in ["set auto-load off", "set debuginfod enabled off", "set auto-solib-add off", "set history save off", "set pagination off", "set target-file-system-kind unix",
                     "set sysroot ./no-libraries", "set solib-search-path ./no-libraries",
                     "set debug-file-directory ./no-libraries"]:
         gdb.execute(command, to_string=True)