Roster the RcStr runtime scaffold

[gunbai-bot]

rt_string_carrier landed in #9256 as a new hand-authored target-language blob but was omitted from gunbc.language_source_scaffold_index. Once #9106 executes the live-tree coverage witness, that omission is exposed as runtime_rust_blobs_are_all_rostered = false.

This adds the missing identity-grain row. Its disposition deliberately reuses runtime_rust_host_intrinsic_kernel_trigger; the authority chain is:

1. rt_string_ops is already rostered under that trigger through runtime_rust_string_ops_row.
2. The trigger is Scaffold { dissolves_to: RealizationDispatch, bind: std.realization_schedule.RealizationPlan WholeDeclaration }: its dissolution condition is realization dispatch, not one blob's private lifecycle.
3. rt_string_carrier states its relationship to that existing blob directly: “Semantics are the free functions' semantics: each method falls back to the function it shadows whenever the flag is false,” after naming char_at, substring, and string_length as the no-carrier entry points it shadows.

The carrier therefore shadows and falls back to the same string-operation authority already governed by this trigger. Reuse avoids inventing a second lifecycle claim for RcStr.

This is a predecessor of #9106, not part of its deletion diff. #9106 makes this live-tree witness execute; landing it first would turn the existing omission into a main-wide red.

Verification:

• current-source release build: cargo build --release -p v1-compiler --bin gunbc --bin v1_src_dag_parse
• parse sweep: 4095 files parse-clean
• focused live-tree claim: PASS runtime_rust_blobs_are_all_rostered