Branch to make i64 paths into bigint for very large environments

[prozacchiwawa]

Note

Medium Risk
Changes environment path computation/encoding in codegen to avoid overflow on large environments, which can affect compiled output across dialect versions. While guarded by stepping/version logic and covered by a regression test, it touches core compilation behavior.

Overview
Fixes an overflow bug in environment path lookup during codegen by switching the internal path accumulator from u64/i64 math to Number/bigint math, and adds a compatibility shim (early_stepping_truncate_to_u64) to preserve pre-cl-26 output encoding.

Introduces the *standard-cl-26* dialect (and bumps MAX_STEPPING to 26) and adds a regression test plus fixtures (test-env-overflow.clsp, test-env-overflow-cl24.hex) that compile/run a large-environment program to validate both the corrected cl-26 behavior and stable legacy cl-24 output.

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