Refactor waspc e2e tests from shell-script DSL to native Haskell steps

[cprecioso]

Previously, each e2e test case was built up as one big bash one-liner
(ShellCommandBuilder produced shell command strings folded with ' && ')
and executed via 'shell'. Snapshot tests additionally executed their
commands at test-tree construction time, before tasty even started.

Now each test case is a list of native Haskell steps:

• Step (ReaderT context IO) replaces the pure Reader shell-string builder.
• Commands are first-class values run via proc (no shell), with explicit
  cwd from the step context, env var modifiers, stdin support, output
  captured for assertions and teed to the per-test log, and process-group
  termination on interruption (now also for regular tests).
• File operations and assertions use System.Directory & friends instead
  of printf/base64/awk/grep/rsync hacks, making steps platform-agnostic.
• Snapshot tests are now single directory-level golden tests
  (tasty-golden Advanced API) that run their steps at test runtime,
  compare current vs golden snapshot dirs reporting all mismatches at
  once, and keep the accept-all and --accept workflows working.
• Per-step log sections and step-aware failure messages make failures
  point at the exact failing step.

Long-running process support (FIXME'd tests) is not implemented yet,
but Command.startCommand/terminate is designed to support it.

See #3404

https://claude.ai/code/session_01HLhBbhP5j1tGKcqPLXu8oF

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