fix(stim-parser)!: follow Stim's tag model

[rafaelha]

Summary

Stim tags are a single opaque string, not a comma-separated list of structured values. This PR updates stim-parser to follow that model and moves PPVM-specific modifier parsing into the extended lowering stage.

• Parse one decoded tag string per instruction, including spaces, commas, parentheses, punctuation, [, and Unicode.
• Support Stim's \n, \r, \B, and \C escapes; reject unknown escapes and raw line breaks.
• Preserve tags on gates, noise, measurements, annotations, MPP, MPAD, and REPEAT blocks.
• Canonically re-encode decoded tags when printing.
• Remove the public Tag / TagParam representation in favor of tag: String (breaking Rust AST change).

Simulation semantics

Tag interpretation is limited to PPVM's reserved carrier instructions:

Instruction	Behavior
S / S_DAG	Exact decoded tag T promotes to T / T-dagger; every other tag is ignored for simulation.
I	Every non-empty tag must parse as a valid R_X, R_Y, R_Z, or U3 modifier; malformed, unknown, or comment-like tags are errors.
I_ERROR	Only exact loss and correlated_loss tags are accepted.
Direct T / T_DAG	Tags are ignored for simulation.
All other instructions	Tags are metadata and do not affect simulation.

This keeps ordinary Stim metadata readable and compatible while preventing misspelled PPVM modifiers on I or I_ERROR from silently becoming comments.

Testing

• cargo test --workspace -q
• cargo clippy -p stim-parser -p ppvm-stim --all-targets -- -D warnings
• Parser tests cover opaque content, every Stim escape, invalid escapes and raw line breaks, empty tags, annotations, REPEAT blocks, and parse/print/reparse behavior across every instruction family.
• Lowering tests cover exact S[T], ignored metadata, strict I modifiers, strict I_ERROR modifiers, and direct T / T_DAG tags.

[github-actions]

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[Copilot]

Pull request overview

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