handle unbalanced braces safely in interpolated text

[UnknownPlatypus]

This fixes 2 edge cases:

• deep {{ stacks was causing panic

thread 'fmt_snapshot' (211935) panicked at markup_fmt/src/parser.rs:2945:21:
attempt to add with overflow
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

• a }} with no matching {{ was treated as an interpolation end, slicing out-of-bounds with get_unchecked in release

thread 'fmt_snapshot' (215456) panicked at /home/thibaut/.rustup/toolchains/stable-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/str/mod.rs:703:22:
unsafe precondition(s) violated: str::get_unchecked requires that the range is within the string slice

This indicates a bug in the program. This Undefined Behavior check is optional, and cannot be relied on for safety.

AI policy

• The pull request description was written manually by me and was not generated by an AI tool or agent.
• I have read and understood the relevant parts of the existing codebase before making these changes.
• I have carefully reviewed and understood all code changes, and ensured that they match the style, architecture, and conventions of the existing codebase.
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