Clarify generation presets and callback contracts

[morluto]

Summary

Clarifies several user-facing contracts that currently disagree with, or are absent from, the implementation:

• renames the preset table's Default column to Balanced fast, distinguishing it from the actual reuse=1, 50-layer, native-canvas defaults
• marks the actual reuse, layer, and core-reuse defaults in --help
• documents --ref-image-size match|max and how each mode scales reference images
• documents callback cancellation and argument lifetime in the public header
• removes the M5-only label from --show, whose availability depends on terminal protocol detection

The generation behavior itself is unchanged.

Validation

• strict C syntax check for main.c — passed
• README fenced-code balance check — 34 fence lines, balanced
• git diff --check — passed