lexer.s turns cached source lines into tokens with stable kinds and error
codes (tokens.inc): 32 keywords, identifiers folded across both PETSCII
letter ranges, decimal/hex/binary numbers to 65535, strings and character
literals with escapes, all operators, and comments. An error reports its
exact column, then the rest of the line is skipped.

An independent host reference lexer and the C128 lexer in x128 both
reproduce the reviewed golden token files: 10 fixtures, 1,119 tokens,
including one fixture per error kind. A case-folding mutation is caught.

Spec 2.2 now lists <= and >=, matching section 8. D119 (proposed): the \n
and \r escapes both produce $0D. The monitor helper now reads RAM bank 0,
because BASIC ROM hides $4000+ after a program returns.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src_load reads a SEQ file from device 8 once into a bank-1 cache (D118).
It enforces the 80-byte line, 599-line, and 18,999-byte limits as bytes
arrive and reports stable codes with exact positions. src_rewind and
src_next_line replay the cache one logical line at a time for every pass.
diskio.s wraps the KERNAL file calls; channels close on every path.

Verified in x128 with a 1581: eight fixtures, including the exact limits,
each limit error, a missing file (DOS 62), and a missing final $0D, all
match host-computed line counts, Fletcher-16 checksums, and positions.
Adds a shared PowerShell test library for x128 scripts.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>