fix(scripts): use gzip -dc instead of zcat for portability

[malteos]

Problem

On macOS (and other BSD systems) /usr/bin/zcat is not GNU zcat — it is the
compress-era tool, which unconditionally appends .Z to its argument and cannot
read gzip files at all:

$ zcat cc-main-2026-jun-jul-aug-host-vertices.paths.gz
zcat: can't stat: cc-main-2026-jun-jul-aug-host-vertices.paths.gz
      (cc-main-2026-jun-jul-aug-host-vertices.paths.gz.Z): No such file or directory

Every zcat in src/script/ is fed .gz input, so the graph exploration and
processing workflows do not run on macOS.

Why the failure is hard to diagnose

In graph_explore_download_webgraph.sh the call is the left-hand side of a pipe
into a while loop:

zcat "$NAME-vertices".paths.gz \
    | while read path; do ... done

The pipeline's exit status is the while loop's, which succeeds trivially on
empty input, so set -e never fires. The script continues, downloads zero vertex
part files, and only fails afterwards at the list_webgraph_files check —
reporting Missing vertices.*.txt.gz () rather than the decompression error that
caused the files to be missing. The reported symptom is several steps removed
from the cause.

The same shape recurs in graph_explore_build_vertex_map.sh, where the iepm
build silently falls through to the mph/fcl path.

Fix

Replace zcat with gzip -dc at all 31 call sites across 6 scripts.

gzip -dc accepts the same inputs as GNU zcat — including .Z, which GNU
zcat also handles — and behaves identically on GNU, BSD and busybox. There is
no behavioural change on Linux; this is purely a portability fix.

The two occurrences in host2domaingraph.sh comments are included so the
documented example commands remain runnable as written.

Alternatives considered

• A zcat() shell function wrapper. Much smaller diff, but it does not
  survive a process boundary: process_webgraph.sh invokes bash -c "zcat ...",
  which would silently fall back to the system zcat unless the function were
  also export -f'd. It also shadows a real command name.
• A $ZCAT variable. Avoids shadowing, but touches the same call sites as
  the literal replacement while adding a definition to keep in sync.

Given equal churn, the direct replacement is the option with nothing to explain
at the call site.

Verification

• bash -n clean on all six changed scripts.
• The previously failing graph_explore_download_webgraph.sh vertex-path
  expansion verified against the live cc-main-2026-jun-jul-aug host graph:
  48 vertex part paths decoded and fetched.
• The graph_explore_build_vertex_map.sh CAT_VERTICES block (both the
  single-file and the host-level $VERTICES/*.txt.gz directory form) verified
  against real host vertex data under set -eo pipefail.

Tested on macOS 26.5 (arm64), GNU bash 5.3.

Note on a pre-existing issue (not addressed here)

Independent of zcat: in graph_explore_build_vertex_map.sh, the iepm path
branches on [ -d $VERTICES ] to handle the host-level multi-file layout, but
the mph/fcl/smph fallback below it (lines 129 and 138) does not — it passes
$VERTICES as though it were a single file. That fallback only runs when the
iepm build fails, so it is not reachable in the normal path. Left alone to keep
this PR to one concern; happy to open a separate issue.