feat(layout): add --only-new incremental placement mode

[tobsch]

What

Adds an incremental mode to config layout. Today config layout re-arranges the whole page (grid_layout). With this PR:

lox config layout config.Loxone            # unchanged: re-layout whole page
lox config layout config.Loxone --only-new # NEW: place only unpositioned blocks

--only-new positions only blocks that have no Px yet and leaves every already-positioned block byte-identical.

Why

After config add, new blocks land at no coordinates while the rest of the page is already laid out by hand. Re-running the full layout throws that hand-work away. Un-positioned blocks also trip the check warning:

⚠ missing canvas layout (Px/Py/Px2/Py2) — Loxone Config will repair this block on first save and may drop wires

--only-new clears exactly those blocks without touching the existing diagram — so the editor won't "repair" (and possibly re-wire) them on first save.

How

incremental_layout() does a small Sugiyama-style layered layout over just the new blocks:

• layer = longest path over the sub-DAG of new blocks (only predecessors that are themselves new count)
• order within a layer by barycenter of predecessor rows (crossing reduction)
• coordinates snapped to the 96-unit editor grid, anchored in the free area to the right of the page's existing content (max Px2 + gap)

Existing blocks are never re-read for placement, so their coordinates are preserved exactly.

Verification

On a real ~500-block house config with 9 freshly-added logic gates (Or/And/Not chain):

• 9 gates placed in clean left→right layers (layer 0 → 1 → 2)
• 498 already-positioned blocks: 0 changed
• config validate → 0 errors
• the 9 missing canvas layout warnings cleared

Adds two unit tests (incremental_places_only_new_in_layers, incremental_noop_when_all_positioned). Full suite green (406 tests).

Notes

• Per-page, like the existing layout commands (--page selects the page; default = first page).
• Byte-clean diffs additionally benefit from the emitter fix in fix(config_edit): byte-clean writes matching Loxone's format #13, but this PR is independent of it.

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