Modernize to Go 1.27 and split the parser

[jenska]

What changed

Go 1.27

• go.mod go 1.26 → go 1.27
• CI matrix 1.26.x → 1.27.x
• README Go badge and install instructions

Standard-library modernization

Applied current idioms across the codebase (verified clean with gopls modernize):

• maps.Copy instead of manual copy loops
• slices.Contains instead of linear search loops
• slices.Clone instead of append([]T(nil), s...)
• min() instead of if-clamp
• strings.Cut instead of IndexByte + slicing
• errors.AsType[*Error] (Go 1.27) instead of errors.As with a separate declaration
• range-over-int in tests; removed a map[bool]string lookup trick in canonical.go

Structural

• Merged the two duplicated size-validation helpers (sizeAllowed in assemble.go, sizeAllowedList in parser.go) into a single sizeAllowed(allowed, sz) with an empty-list guard.
• Split the 1,393-line internal/asm/parser.go into focused files — pure code movement, no behavior change:
  • parser.go (272) — Parser type, entry points, token stream, register helpers
  • parser_labels.go — local/global label handling
  • parser_stmt.go — statements, constants, instructions, padding
  • parser_macro.go — macro expansion
  • parser_operand.go — form/operand/size parsing
  • parser_ea.go — effective-address parsing
• gofmt the previously unformatted additional_api_test.go.

Notes for reviewers

• Public API is unchanged.
• The m68kdasm dependency mentioned in the original request was intentionally left out: it is not a dependency of this project, and m68kdasm v1.1.0 in fact depends on m68kasm.
• Verification: go build ./..., go vet ./..., go test ./... (incl. tests/e2e), benchmark compilation, and a bin/srec/elf CLI smoke test all pass.

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