refactor(reg-vm): extract IR/bytecode model into reg_vm/model.rs

Pure code-movement (no logic/behavior changes). mod.rs 11,872 -> 10,183.
Moved a contiguous span (RegUnit+impl, RegFunction+impl, RegInstr, RegIntrinsic,
RegIntCompare, Reg, LoopPatch, MatchFailurePatch, profiling model
CallSiteFeedback/FunctionProfile/MonoState/record_call_site + consts, and both
cfg-gated OsrTrigger defs) into reg_vm/model.rs (1,692).

mod.rs re-exports via `pub(crate) use model::*;` so every sibling module's
`use super::*` still resolves these. Moved items widened module-private ->
pub(crate) (crate-internal; bodies byte-identical to HEAD). mod.rs gains only
`mod model;` + the re-export.

Verified: cargo test -p rsscript --no-run (default + native-jit) clean; clippy
-p rsscript --tests -D warnings clean; reg_vm::tests 17/0; runtime
vm_eval_parity 130/0.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: olwang