Reduce recursive verifier random coin overhead

The MASM recursive verifier now matches the v0.26 transcript layout, but it pays for Rust-style buffered random coin handling in places where MASM can use a shorter path. The transcript order must stay the same; the MASM path should do less stack and memory work.

The main target is `crates/lib/core/asm/stark/random_coin.masm`. `observe_felt`, `flush_buffer`, and their callers can be cheaper for the recursive verifier's known transcript flow. The same cleanup applies where `crates/lib/core/asm/sys/vm/public_inputs.masm` calls `observe_felt` for fixed public inputs, kernel digest felts, the instance shape, and the main-trace commitment.

In `init_seed`, the loop that computes `g^(2^(32 - log_lde))` can keep the count deeper to reduce stack churn.

Keep the Rust code unchanged unless correctness requires it. Keep the transcript elements identical. This is done when recursive verification still passes and these MASM paths do less work.

Source comments

- `random_coin.masm` buffered absorption, [https://github.com/0xMiden/miden-vm/pull/3228#discussion_r3374342180](https://github.com/0xMiden/miden-vm/pull/3228#discussion_r3374342180)
- `random_coin.masm` LDE coset loop, [https://github.com/0xMiden/miden-vm/pull/3228#discussion_r3374348220](https://github.com/0xMiden/miden-vm/pull/3228#discussion_r3374348220)
- `public_inputs.masm` random coin efficiency, [https://github.com/0xMiden/miden-vm/pull/3228#discussion_r3374593017](https://github.com/0xMiden/miden-vm/pull/3228#discussion_r3374593017)
- `random_coin.masm` instance shape and main commitment, [https://github.com/0xMiden/miden-vm/pull/3228#discussion_r3374596108](https://github.com/0xMiden/miden-vm/pull/3228#discussion_r3374596108)