fix(rust): remove double-increment of cb_708 counter

The cb_708 counter was being incremented twice for each CEA-708 data block:
1. In do_cb_dtvcc_rust() in Rust (src/rust/src/lib.rs)
2. In do_cb() in C (src/lib_ccx/ccx_decoders_common.c)

Since FTS calculation uses cb_708 (fts = fts_now + fts_global + cb_708 * 1001 / 30),
the double-increment caused timestamps to advance ~2x as fast as expected,
resulting in incorrect milliseconds in start timestamps.

This fix removes the increment from the Rust code since the C code already
handles it in do_cb().

Fixes timestamp issues reported in PR #1782 tests where start times like
00:00:20,688 were incorrectly output as 00:00:20,737.

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

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

Author: cfsmp3

src/rust/src/lib.rs

@@ -529,7 +529,9 @@ pub fn do_cb_dtvcc_rust(ctx: &mut lib_cc_decode, dtvcc: &mut DtvccRust, cc_block
                 if timeok && ctx.write_format != ccx_output_format::CCX_OF_RAW {
                     dtvcc.process_cc_data(cc_valid, cc_type, cc_block[1], cc_block[2]);
                 }
-                unsafe { cb_708 += 1 }
+                // Note: cb_708 is incremented by the C code in do_cb(), not here.
+                // Previously incrementing here caused a double-increment bug that
+                // resulted in incorrect start timestamps.
             }
             _ => warn!("Invalid cc_type"),
         }