fix(core): justify-content honors min-resolved root main size (closes #165)

[zhijiewong]

What

A bare-auto root container resolved its main-axis size from min-width/min-height
after positioning its children, so justify-content collapsed them to
flex-start at 0 (root width became 50 but the child stayed at left: 0).
Yoga distributes within the resolved size.

root.setFlexDirection('row'); root.setJustifyContent('center'); root.setMinWidth(50);
// child 20×20 → was left:0, now left:15  (Yoga parity)

Fix

At the positioning step only, for a single-line, forward-direction bare-auto
root main axis, both engines position against max(innerMain, clamp(content, min, max) − pads):

• Classic (index.ts + main-axis.ts): threads a rootMainAuto flag
  (via the now-exported axisIsBareZero) into the flex pipeline; substitutes at
  positionItemsInLine, gated rootMainAuto && noWrap && !isReverse.
• Spineless (flex-grammar.ts): feeds the root's min/max main fields into
  emitJustifiedMainPos under the existing rootAxisIsBareZero structural gate
  (forward, no-wrap).

The max(…) guard makes the change a no-op for every concrete-size container.
The two engines are kept byte-identical; the differential fuzzer confirms it
(its imperative reference was also updated to pass rootMainAuto, restoring it
as a real guard for this code).

Tests

• 7 unit tests in layout.test.ts (classic + spineless paths, multi-child
  space-between, shrink-wrap regression, and a wrap-deferral consistency guard).
• Reinstated the 2 fixtures PR fix(core): root layout offsets by own margin (closes #163) #166 deferred:
  row-min-width-and-margin, column-min-height-and-margin.
• pnpm run ci green (lint → build → typecheck → test 1770 → test:differential 976).

Scope / follow-up

No-wrap, forward-direction main axis only. The cross-axis (align-items) analog
and the wrap-justify case are deferred and tracked in #172 — both kept
consistent across engines.

Closes #165.

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