Add :powerio parser backend (hard dependency on PowerIO) #56

Route parse_file(path; backend=:powerio) through the PowerIO Rust core, making PowerIO a hard dependency of PowerDiff. PowerIO stays raw and lossless; PowerDiff normalizes (the existing _normalize_* / _finish_parse tail), so a ParsedCase from :powerio is indistinguishable from :native.

Status: wired on mk/backends.

• import PowerIO; PowerIO in [deps], pinned via [sources] (git) until it registers in General — then drop [sources] and add [compat].
• _load_powerio_network + _parsedcase_from_powerio + _powerio_cost_row in src/parser.jl; :native stays the default; :powerio needs a file path (the IO method rejects it).
• caseio→powerio rename throughout; docs/caseio-integration.md → docs/powerio-integration.md.
• Parity test in test/test_parser_parity.jl: :powerio == :native field-for-field on case5 / case14 / case30, gated on PowerIO.library_available(). Verified 672/672 locally.

Ordering / blocked on:

• Push the PowerIO accessor branch named in [sources].rev before this branch's CI runs.
• A registered PowerDiff cannot ship until PowerIO is registered in General (Self-hosted artifact distribution for libpowerio_capi → register in General eigenergy/PowerIO.jl#8).

See docs/powerio-integration.md for the field-by-field contract.