Fix pack_write's O(n^2) dedup + correctness bug; document mount table O(n^2)

A audit for other instances of the file-index O(n^2) shape (fixed
previously via the persistent treap) found two more real issues:

1. pack_write's Section 9.2 exact-duplicate elimination was a linear scan
   of every previously-seen (hash, size) pair per entry -- O(n^2) total,
   invisible in the existing benchmark because its identical-content test
   files made every scan match on the first comparison. Measured with
   unique content instead: 80,000 entries took 1.74s, with a 20,000->80,000
   step showing 15.6x for a 4x-N step, matching O(n^2)'s 16x prediction.
   The same scan also trusted a (hash, size) match without ever comparing
   actual bytes -- a latent correctness bug, since FNV-1a64 is explicitly
   not collision-resistant. Fixed both at once with an open-addressing hash
   table (load factor 1/2, linear probing) plus a memcmp verification
   before ever reusing a data_off. Post-fix: 80,000 entries in 0.044s
   (39.6x faster), ratio drops to 3.35x (consistent with O(n)).

   Covered permanently by two new/extended tests: a white-box assertion in
   test_pack_overlay.c that duplicate-content entries share one data_off
   and distinct-content entries do not, and a new
   tests/test_pack_write_perf.c regression tripwire against 10,000 unique
   entries.

2. vfs.c's mount table uses the same full-array-copy-per-write pattern the
   file index used to, confirmed O(n^2) via a new bench/bench.c category
   (500/2,000/8,000 mounts, both 4x-N steps showing 15-20x). Deliberately
   NOT rewritten: mount points are created by a program's own source code,
   not workload-driven, so realistic mount counts never reach the scale
   that made the file index's O(n^2) a real problem. Documented with full
   reasoning in BENCH.md and CLAUDE.md rather than silently left as an
   undocumented gap.

Also fixes a real CI gap the new tests exposed: ci.yml's sanitizer-build
steps never passed -D_GNU_SOURCE when compiling test files (only the
library .o's got it), which was harmless while no test included
internal.h and became a link failure once two did (internal.h needs
_GNU_SOURCE for pthread_rwlock_t). And documents, in CONTRIBUTING.md and
CLAUDE.md, a sandbox flake observed directly during this work's own
sanitizer runs: ASan/UBSan test binaries occasionally fail to start with
AddressSanitizer:DEADLYSIGNAL (sometimes looping rather than exiting),
non-deterministically hitting different unrelated binaries across runs --
a startup race, not a memory-safety bug, confirmed by clean passes on
retry; sanitizer runs in such an environment should be timeout-wrapped.

BENCH.md's "After" table and Appendix B are replaced with the current,
complete 54-measurement bench/bench.c run (the original 45 plus the new
mount-scaling category); the pre-fix 45-measurement "Before" table is kept
as the historical record, per this project's documentation standard.

Verified: make test (all 6 binaries, including the 2 new/changed), a clean
make all, and repeated ASan+UBSan runs (0 real findings; the DEADLYSIGNAL
flake above was observed and correctly distinguished from a real finding
by re-running until a clean pass). TSan could not be run in this sandbox
(pre-existing, documented environment limitation).

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01UqJpkdJ6Njnt1pw3CbghzB

tests/test_pack_overlay.c

@@ -10,6 +10,7 @@
 #include <unistd.h>
 
 #include "packfs.h"
+#include "internal.h" /* white-box: verifies Section 9.2 dedup actually shares data_off, not just "doesn't crash" */
 #include "test_harness.h"
 
 static char *tmp_pack_path(void) {
@@ -51,6 +52,28 @@ int main(void) {
 
     CHECK_EQ_INT(vfs_sync(v, "/"), VFS_OK); /* compaction */
 
+    /* Section 9.2 dedup, actually verified (not just exercised): /a.txt
+     * and /dup.txt have identical content and must share one data_off
+     * in the compacted pack; /b.txt has different content and must not
+     * share either's. Catches both "dedup silently stopped happening"
+     * (a real, previously-unasserted gap) and "dedup over-matched two
+     * different files onto the same bytes" (the hash-collision-without-
+     * memcmp-verification bug fixed alongside the O(n^2) compaction
+     * cost — see pack.c's DedupSlot comment). */
+    {
+        Pack *p = NULL; int lerr = 0;
+        CHECK_EQ_INT(pack_load(pack_path, &p, &lerr), 0);
+        if (p) {
+            const PackIndexEntry *ea = NULL, *edup = NULL, *eb = NULL;
+            CHECK(pack_find(p, "/a.txt", &ea));
+            CHECK(pack_find(p, "/dup.txt", &edup));
+            CHECK(pack_find(p, "/b.txt", &eb));
+            if (ea && edup) CHECK_EQ_INT(ea->data_off, edup->data_off);
+            if (ea && eb) CHECK(ea->data_off != eb->data_off);
+            pack_close(p);
+        }
+    }
+
     /* read-only open straight from the freshly-compacted pack */
     f = vfs_open(v, "/a.txt", VFS_O_RDONLY, &err);
     CHECK(f != NULL);