feat(emit): corporate theming as data — palette/typography tokens in YAML (#64)

[danielPoloWork]

Problem (#64)

Corporate identity was hardcoded three times over: emit_pptx RGB constants + font.name = "Calibri", emit_docx re-declaring the same palette, emit_svg carrying a third THEMES dict. An enterprise consumer could not render a board deck in their corporate palette/typeface without editing three tools — violating "adding … is editing a validated YAML file, never a special case in code", and blocking the realistic GTM ask: "make it look like our template."

Fix — theme tokens as data

• New os/themes/<name>.yaml — design tokens (font + accent/body/muted/card/background), one file per theme. professional.yaml is byte-for-byte the old constants (default output unchanged); scientific.yaml moves the emit_svg variant to data. Adding a corporate theme = adding one YAML file.
• New tools/themes.py — cached loader + rgb helper. The amber "to verify" color is deliberately NOT a token: it is the grounding signal (RFC-0001 §6), reserved in code and identical across themes — a theme that camouflages assumptions defeats the safety model.
• theme: param on the presentation / mindmap / architecture registries — validated by the existing gate_registry_params machinery, zero new gate code. The deck / graph / topology IRs carry the resolved theme name, so emitters stay IR-driven. The infographic keeps visual_style as its selector; its palette now loads from the same theme files.
• emit_pptx / emit_docx / emit_svg read tokens instead of constants; the pptx typeface comes from the theme's font token.

Verification

• Default output byte-identical on 33 SVG baselines (all 11 refs × infographic/mindmap/topology), diff'd clean.
• A second theme demonstrably restyles from one YAML file: scientific accent 0B3D5C verified inside the emitted .pptx binary and in the SVG; amber B86B00 present in both themes.
• Unknown theme (neon) rejected by the registry gate; deck-IR theme stamp covered; os/deck-ir/_schema.md updated.
• 97/97 tests green.

The --template corporate.pptx slide-master inheritance is the issue's stretch goal — left for a separate PR as suggested.

Closes #64

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