docs(plugin): update F-Sy-H guide

[ss-o]

Summary

• document the current F-Sy-H loading and unload contract
• replace removed fast-theme and FAST_WORK_DIR interfaces with fsh_theme and :fsh:config
• document theme metadata, accepted colour values, canonical fallbacks, overlays, paths, and optional secondary themes

Validation

• pnpm validate:frontmatter
• pnpm validate:code-fences
• pnpm lint
• pnpm build:en
• LLM corpus validation: 84 artifacts from 78 canonical documents

Related: z-shell/F-Sy-H#54
Closes z-shell/F-Sy-H#91
Security advisory: z-shell/F-Sy-H#GHSA-33c2-cgpq-8gpc

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[ss-o]

Heads-up: this documents the pre-v2 F-Sy-H API, which changed on main today.

z-shell/F-Sy-H#101 ("refactor!: align F-Sy-H with plugin standard v2") landed
and renamed the user-facing interface. The additions in this pull request use
the old names:

documented here	current on F-Sy-H main
fast-theme	fsh_theme
FAST_WORK_DIR	zstyle ':fsh:config' work-dir ...
ZSH_HIGHLIGHT_MAXLENGTH	zstyle ':fsh:config' max-length ...
FAST_THEME_MANAGER_DISABLED=1	zstyle ':fsh:config' theme-manager disabled
the f-sy-h alias	removed

F-Sy-H/README.md:108-125 carries the full migration list, and states that the
legacy names are deliberately not retained as a second compatibility interface.

The sentence about current_theme.zsh and theme_overlay.zsh being sourced
during loading also needs review: the v2 README says executable legacy theme
cache files are no longer loaded and a theme must be reapplied with fsh_theme.

Worth updating before merge so the wiki does not ship instructions that fail on
the current release.