chore: merge Marstek Venus E Gen 3.0 template (BC)

[andig]

Venus E Gen 3.0 only differs from Venus E, E Gen 2.0 and Venus C in the power and soc registers. Both templates are merged, the register map is picked by a new generation param.

• marstek-venus-e covers marstek-venus-e-v3, so the old template name still resolves
• generation has no default and is required, since guessing it would silently read the wrong registers. Existing configs have to add it
• capacity and maxchargepower defaults are unchanged, both templates already used 5.12 kWh / 2500 W

🤖 Generated with Claude Code

/cc @webalexeu

[sourcery-ai]

Hey - I've left some high level feedback:

• Consider using more descriptive generation values (e.g. "2.0", "3.0" or "gen2", "gen3") instead of plain numeric strings to avoid confusion with other potential revisions.
• The conditional logic for power and SOC registers is now duplicated inline; you might make this more maintainable by moving generation-specific register maps into a small helper/include instead of sprinkling if eq .generation blocks in the render section.

Prompt for AI Agents

Please address the comments from this code review:

## Overall Comments
- Consider using more descriptive generation values (e.g. `"2.0"`, `"3.0"` or `"gen2"`, `"gen3"`) instead of plain numeric strings to avoid confusion with other potential revisions.
- The conditional logic for power and SOC registers is now duplicated inline; you might make this more maintainable by moving generation-specific register maps into a small helper/include instead of sprinkling `if eq .generation` blocks in the render section.

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