fix(ui): drop the reasoning body's left rail, restore upstream geometry

[Astro-Han]

Why

The expanded 深度思考 body gained a 2px link-tinted left rail in #2645. Two problems:

1. It is not the upstream Astryx design. The ejected ChatReasoning atoms own the body's reading geometry already — secondary text color (xv1l7n4), 8px top padding (x1xye8es), 22px inline-start indent (x1f43n9v, calc(16px + spacing-1-5)) — and none of the ejected atoms draws a border. Upstream differentiates reasoning by color + indent alone.
2. Half of the added rule was dead code. The atoms' stacked :not(#\#) specificity beats the product class, so padding-top: 2px and padding-inline-start: 12px in padding: 2px 0 4px 12px never applied; the visible line-to-text gap was the atom's 22px, not the authored 12px.

The saturated 32%-link rail also visually outweighed the muted text it framed — the loudest element in the block was the decoration.

What

Restore the pre-#2645 rule on .maka-chat-reasoning-content (white-space: pre-wrap; word-break: break-word; only) and let the upstream atoms own layout. Net diff vs. before #2645 is zero; the comment now records why no rail.

Validation

• node --test dist/__tests__/chat-turn-answer-identity.test.js — 8/8 pass, including the test that pins white-space: pre-wrap on this rule.
• No test, story, or e2e fixture references the inset declarations (grep: border-inline-start, margin-inline-start: 22px only matched this rule).

AI-assisted (Maka): investigation, diff, and this description; geometry claims verified against the compiled atoms in @astryxdesign/core dist.

[M4n5ter]

Reviewed exact head 11c0b0b18d478f4fbae2c6c307c9d5dd32ecfb9c: no P0-P3 findings.

This change returns .maka-chat-reasoning-content to the pre-#2645 ownership boundary: Maka keeps only whitespace preservation and long-token wrapping, while the ejected Astryx atoms remain the sole owner of the body's 8px top padding and 22px inline-start padding. The removed product rule's top and inline-start padding were already overridden by those higher-specificity atoms; deleting its effective margin, border, and bottom padding therefore removes the duplicate layout authority without changing the reasoning DOM or interaction contract.

The exact-head hosted checks are green. I also validated the current-main merge result: it changes only this CSS rule, preserves the reviewed blob, builds Core and UI, and passes all 247 UI tests. Biome and git diff --check are clean; there are no existing reviews or review threads.

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