Require bash 4+ explicitly instead of failing silently under /bin/bash

[systemjack]

localdev-mounts.sh uses declare -A for mount-target collision detection, so it
needs bash 4.0+. Every launcher declared #!/bin/bash, which on macOS is bash
3.2.57 -- the one interpreter that cannot run it.

The failure was silent and wrong rather than loud. Under 3.2 declare -A fails,
USED_TARGETS degrades to an indexed array, subscript expansion is evaluated as
arithmetic, and .localdev-mounts.toml entries are dropped with no error. The
container then comes up missing mounts the user believes are present -- observed
as a workspace whose read-only reference repo simply was not there, with nothing
in the output to say so beyond two stray declare: -A: invalid option lines.

The same breakage silently disabled the collision guard itself, so two host
directories claiming one container target went undetected on macOS for as long
as the file-mount feature has existed.

Two changes:

• Shebangs become #!/usr/bin/env bash in all four launchers and the library, so
  a PATH-resolved bash 4+ (Homebrew's, typically) is preferred over /bin/bash.
• The library asserts BASH_VERSINFO >= 4 and exits with an actionable message
  naming the running version and interpreter. #!/usr/bin/env bash alone would
  still fail silently for anyone without a modern bash on PATH, and silently
  dropping mounts is not an acceptable failure mode.

Verified: launched by name, the .localdev-mounts.toml entry is now present in the
podman args as :ro; invoked as /bin/bash localdev it aborts with the version
error rather than proceeding; and the collision guard now correctly aborts on two
mounts sharing a target, which it never did on macOS before.

This affects every macOS host, since /bin/bash there is 3.2 -- anyone relying on
.localdev-mounts.toml has been silently losing mounts.

Also documents the requirement in README Prerequisites. The runtime guard tells
you after the fact; a macOS user needs to know before their first launch that
brew install bash is a setup step, not an optional nicety.