Add core.fs stdlib: comparison, range, numeric helpers

[sigilante]

Summary

First cut of North's Forth-defined standard library, layered on top of the interpreter primitives.

desk/lib/core.fs — 9 words across three groups:

• Comparison shorthands: <=, >=, 0<>, U<=, U>= — defined in terms of existing primitives (>, <, 0=, NOT, U<, U>).
• Range test: WITHIN ( n lo hi -- f ) — true iff lo <= n < hi, standard semantics.
• Numeric helpers: SQUARED, CUBED.
• Debug: ? — fetch-and-print (@ .).

Uses paren comments throughout so the file can be flattened to a single line for tooling that does not preserve newlines (notably the test harness).

Test harness wiring

tests/test-north.sh now reads core.fs, escapes it into a CORE_SRC shell variable, and prepends it to each core-stdlib test's Forth source. This exercises both the parse/compile path (the : definitions) and the runtime behavior on every run — unlike the existing token-list tests, which bypass parsing.

Added a === Core stdlib === section with 24 new tests covering boundaries (equal, below, above) for each comparison, all five regions for WITHIN, zero-and-nonzero cases for the numeric helpers, and an ALLOT/!/? sequence for ?.

Test plan

• bash tests/test-north.sh — 341 passed, 0 failed (was 317 before)
• Manual smoke test in the shoe REPL: INCLUDE /lib/core then exercise a few words — requires copying core.fs onto the ship desk

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