Add 69 tests covering previously untested transpiler features

[associate-1]

Summary

• Systematic gap analysis of implemented features vs existing test coverage identified 10+ feature areas with zero or near-zero tests
• Adds 54 new e2e tests across 5 new test files and 15 new lexer unit tests, bringing the total from ~289 to ~358
• All new tests pass alongside the full existing suite

New test coverage

File	Tests	Covers
e2e_intrinsics_test.go	13	LONGPROD, LONGDIV, LONGSUM, LONGDIFF, NORMALISE, SHIFTRIGHT, SHIFTLEFT
e2e_retypes_test.go	6	VAL INT RETYPES (float32→int), VAL [2]INT RETYPES (float64→int pair), same-name shadowing
e2e_params_test.go	6	RESULT qualifier, fixed-size array params, shared-type params, VAL []BYTE with string arg
e2e_strings_test.go	5	VAL []BYTE abbreviation, print.string, print.newline, string escapes
e2e_misc_test.go	24	SKIP, STOP, modulo, ALT boolean guards, MOSTNEG/MOSTPOS REAL32/REAL64, shorthand slice, Go reserved word escaping, multi-line expressions, recursive functions, go vet cleanness, and more
lexer_test2_test.go	15	All 33+ keywords, paren/bracket depth suppression, continuation operators, string/byte literals, send/receive/ampersand/semicolon tokens, line/column tracking

See TEST_COVERAGE_IMPROVEMENTS.md for the full detailed breakdown.

Test plan

• go test ./... passes (all 358 tests)
• No changes to production code — tests only
• Each previously-untested feature now has at least one e2e test verifying transpile → compile → run → correct output

🤖 Generated with Claude Code