When network is down, the first git ls-remote or HTTP fetch to fail with
a clearly network-related error (timeout, DNS, connection refused) sets a
global `config._network_available = False`. All subsequent operations
check this flag and skip instantly instead of each waiting for their own
timeout (previously: N deps × 5s = 50s wasted blocking).

Only true network errors trigger the flag. Authentication failures (SSH
key rejected, HTTP 401/403) and HTTP 404 are explicitly excluded so that
misconfigured credentials don't silently disable all network access.

Guard points:
- Git.init_commit_hash: skip ls-remote if flag set
- Git.fetch_origin: skip fetch/pull if flag set
- Git.clone_or_pull: skip pull (use cached source) if flag set;
  fail clearly if clone needed but network unavailable
- _fetch_package: skip HTTP download if flag set

The `is_network_error(e)` classifier lives in mama/util.py and handles
subprocess.TimeoutExpired, urllib URLError/HTTPError, socket errors,
OSError with network errno, and git error message patterns.

Behavior by command:
- `mama build` with full cache: zero network ops, zero cost
- `mama build` first time: first dep times out (5s), flag set, rest
  fail fast with "network unavailable" message
- `mama update`: first dep times out, flag set, rest use cached state

https://claude.ai/code/session_01S5uF8PBFkxx5GhSBBTkZvK

…ange on repeat mama update)

…ti-thread safe)

…wraps, one-liner ifs)

…ness check)

…n Popen formatting

…-dashes

…erves cache

…e set

…se docstrings)

…strings

…nd refactor learnings

…_shim, trim docstrings)

Plain `mama build` (without `update` and without `noart`) now routes
through try_load_cached_shim when a valid shim marker exists. This skips
both the ls-remote probe and the cached-zip re-extraction that
_try_artifactory_shim previously did on every invocation. Update mode
still bypasses the cached path so a re-extract happens; noart still
ls-remotes for staleness.

Captured the build-vs-update contract in docs/build_behavior.md so the
intended behaviour does not drift again.