feat(rust): Add persistent DtvccRust context for CEA-708 decoder (Phase 1)

This is Phase 1 of the fix for issue CCExtractor#1499. It adds the Rust-side
infrastructure for a persistent CEA-708 decoder context without
modifying any C code, ensuring backward compatibility.

Problem:
The current Rust CEA-708 decoder creates a new Dtvcc struct on every
call to ccxr_process_cc_data(), causing all state to be reset. This
breaks stateful caption processing.

Solution:
Add a new DtvccRust struct that:
- Owns its decoder state (rather than borrowing from C)
- Persists across processing calls
- Is managed via FFI functions callable from C

Changes:
- Add DtvccRust struct in decoder/mod.rs with owned decoders
- Add CCX_DTVCC_MAX_SERVICES constant (63)
- Add FFI functions in lib.rs:
  - ccxr_dtvcc_init(): Create persistent context
  - ccxr_dtvcc_free(): Free context and all owned memory
  - ccxr_dtvcc_set_encoder(): Set encoder (not available at init)
  - ccxr_dtvcc_process_data(): Process CC data
  - ccxr_flush_active_decoders(): Flush all active decoders
  - ccxr_dtvcc_is_active(): Check if context is active
- Add unit tests for DtvccRust
- Use heap allocation for large structs to avoid stack overflow

The existing Dtvcc struct and ccxr_process_cc_data() remain unchanged
for backward compatibility. Phase 2-3 will add C header declarations
and modify C code to use the new functions.

Fixes: CCExtractor#1499 (partial)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: cfsmp3