Signed PHIR register size of usize::MAX overflows the width check (debug panic, release accepts) #1341

A signed classical register declared with size equal to usize::MAX overflows the register-width check.

Reproduction

src = {"format": "PHIR/JSON", "version": "0.1.0", "ops": [
    {"data": "cvar_define", "data_type": "i64", "variable": "a", "size": 18446744073709551615},
    {"cop": "=", "args": [5], "returns": ["a"]},
]}
pecos_rslib.run_phir_sim(json.dumps(src))

In a debug build of pecos_rslib, this panics: attempt to add with overflow at crates/pecos-phir-json/src/v0_1/environment.rs:617 (#1314 branch).

Cause

Environment::validate_variable_size (crates/pecos-phir-json/src/v0_1/environment.rs, ~589 on dev) computes needed = size + 1 for signed types. For size == usize::MAX:

• debug builds panic;
• release builds (including the Python wheel) wrap needed to 0, so the check passes and the declaration is accepted, although the size cannot fit any backing type.

The Python interpreter rejects the same declaration, since Python integers do not overflow, so the paths disagree.

Direction

Use size.checked_add(1) and treat None as "does not fit". The existing "use at most N bits" error then covers it. Add a test with size: usize::MAX for a signed type on native, the Rust interpreter and the JSON-to-module converter.

Found during the review of the #1314 work.