validate-features.mjs: warn when epic.order has gaps（校验器加 epic.order 数值连续性警告）

[Kaka-cheaper]

Goal

Add a heuristic warning to the validator when epic.order values have gaps (e.g. [0, 1, 3] skipping 2) or duplicates beyond what's allowed.

Why

Currently nothing tells the user "you wrote order: 0, 1, 3 — is the missing 2 intentional?". Same-stage epics should share an order; non-sequential gaps are usually mistakes.

Scope (what is in)

• Detect: order values present, find min and max, list missing integers
• Detect: more than 4 epics share one order (likely under-stratified)
• Add to existing warning system (don't fail validation, just warn)
• Output: ⚠ epic.order skips value(s): [2] and ⚠ 5 epics share order=0 — likely too coarse

Out of scope

• Auto-fix
• New error level (must be a warning, not error)

Files likely involved

• scripts/validate-features.mjs (single file)

Acceptance criteria

• Running validator on a fixture with order: [0, 1, 3] prints the gap warning
• No false positive when all-zero order (everything in one stage is OK)
• Existing tests still pass

Estimated effort

~1-2 hours / 1 file

Hints

The existing file-level warnings near line 600-700 (search for 严重不平衡 or 偏好) show the pattern. Reuse pushWarning(...).

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