Update tiny16se spec for assembler features (v0.2)

[rafael84]

Motivation

• Make the tiny16se spec concise and focus on the language semantics the compiler relies on while documenting modern assembler capabilities.
• Encourage the compiler to emit clearer assembly by leveraging assembler features such as macros, constants/expressions, sections, and data directives.

Description

• Rewrite specs/tiny16se.txt to v0.2 with a shorter, focused specification emphasizing core language semantics and calling conventions.
• Add guidance for assembler integration including use of .macro helpers, local labels, NAME = expression constants, and expression support in immediates.
• Document section .code / section .data, DB, ORG, TIMES, and .include usage and include a concise lowering example showing macro-based code generation (FP_ENTER/FP_LEAVE).
• Remove the long, low-level lowering rules and large examples in favor of a compact, maintainable spec that the compiler can target directly.

Testing

• No automated tests were run because this is a documentation-only change and does not modify code or behavior of the toolchain.

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Codex Task