vfs: scope hook to mount, fix negcache to handle backing entries

Three bugs in the PE-side bridge collided to crash NSPBR ~54s after
launch with an AV in NPCWaterAIFix.dll. Root cause: the bridge said
STATUS_OBJECT_NAME_NOT_FOUND for files that exist on disk but are not
in the static index, masquerading as ground truth.

1. load_index() filtered every entry whose real_path didn't start
   with '/'. That dropped every is_backing=true entry — for NSPBR,
   the entire vanilla Skyrim base layer (Skyrim.esm, all base BSAs,
   Video/BGS_Logo.bik). Now accept both forms; backing entries get
   real_nt=NULL and exist only for query / negcache logic.

2. The "open negcache" comment claimed the index was authoritative
   for the FUSE mount tree. With (1), it is not. Comment rewritten;
   the comment about attr_cache_invalidate (which never existed) is
   also corrected to point at attr_cache_mark_exists.

3. Hook_NtCreateFile and Hook_NtOpenFile returned ENOENT on
   `!attr_cache_lookup() || !cexists` — the "no entry at all" case
   got the same treatment as a confirmed-negative cache hit. Plugin
   code that calls CreateFile without a preceding GetFileAttributes
   would never reach Real_NtCreateFile. Now mirrors what
   try_kernel32_attr_hit already did: only ENOENT on confirmed
   negatives; pure cache miss falls through to Real_, which serves
   correctly via Wine + FUSE.

Also includes the previously-uncommitted mount-scope fast gate so
out-of-mount paths (system DLLs, registry-backed objects, unrelated
drives) bypass the hook entirely with a single prefix compare on the
input string — no normalization, no cache lookup, no chance of a
wrong answer.

Verified by a self-contained 44-assertion C reproduction at
/home/luke/builds/fluorine-vfs-tests/test_negcache_logic.c (six
scenarios: backing-filter exclusion, first-open without pre-query,
pre-query masking, SKSE plugin probes, post-fix Skyrim recovery,
nt_path_to_key edge cases).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

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