Refine tiny16se data forms and demo layout

[rafael84]

Motivation

• Support richer and more ergonomic data emission in tiny16se by allowing named data regions, optional addresses (ORG), and multiple data blocks per program.
• Clarify how strings and byte blobs are represented and how data labels can be referenced from code.
• Make the demo source match the new data semantics and be easier to read and assemble.

Description

• Bump language version to tiny16se v0.5 and add string as a token and atom variant in the lexical/syntax sections.
• Introduce the top-level (data name [addr16] form...) form with semantics that emit a .data section, optional ORG, and an emitted label for name, plus add (db ...) and clarify that strings expand to bytes with no terminator and (repeat ...) support.
• Document that data labels may be referenced from code and add compile-time helpers hi/lo and memory primitive notes to the spec.
• Restructure examples/demo.se to use multiple labeled (data ...) blocks (initialized, last-frame, sprite-data, tile-smiley, oam-data, palette), replace bytes with db, use repeat for zeroed regions, and split the previous combined data region into separate named regions.

Testing

• No automated tests were executed for these changes.

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Codex Task