test: add REPL dispatch function tests

[rocketman-code]

Context

Parent: #160

10 of 12 dispatch_*() functions in repl.rs have zero direct tests. The 77 existing REPL tests all cover Command::parse() and parse_flags() — they verify inputs are parsed correctly but never verify that dispatch functions consume those inputs correctly. Three v0.4.0 bugs lived here (#143, #144, #147).

Expected Behavior

Integration tests that construct a real Session, call dispatch functions with various flag combinations, and assert on the output.

Scope

• Add tests for dispatch_trace(), dispatch_chain(), dispatch_cut(), dispatch_diff(), dispatch_packages(), dispatch_imports(), dispatch_importers(), dispatch_info(), dispatch_show()
• Each dispatch function tested with at minimum:
  • Default options (no flags)
  • --json flag (assert output is valid JSON)
  • --top N flag (assert output is truncated)
  • --include-dynamic flag (assert dynamic imports included)
  • Invalid/missing target (assert error message)
• Use captured stdout (or return values) to assert output format
• Small fixture project for Session construction