Type AST: migrate Parameter.typ

[frendsick]

Parent PRD

#140

What to build

Switch Parameter.typ from str to Type. Convert at boundaries: parse incoming source/JSON to Type on construction; format back to str wherever consumers still need strings. All other string-typed slots remain unchanged.

Touch sites: compiler/common.casa (Parameter, make_param, make_named_param), every consumer of Parameter.typ across compiler/typechecker.casa, compiler/syntax.casa, compiler/bytecode.casa, compiler/error.casa, compiler/pattern.casa.

Acceptance criteria

• Parameter.typ: Type
• All consumers compile, format-via-Type.format at string boundaries
• No regressions: tests/test_examples.sh + tests/test_compiler.sh (run < /dev/null for headless lsp)
• Existing string helpers still in place (cleanup deferred to Type AST: replace string helpers with Type methods #166)

Blocked by

• Blocked by Type AST: foundation enum + parser + printer + round-trip tests #161

User stories addressed

Migration step 2 in parent PRD: smallest slot first to validate boundary pattern.