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askrene: clamp impression-adjusted min/max to channel capacity

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@Lagrang3 已经在做这个了。

开始于 2026年8月4日。

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描述

Follow-up from #9150.

layer_apply_constraints() applies impressions without bounding the result:

  • reverse direction: min and max both grow by imp->amount, saturating at UINT64_MAX, with no clamp to the channel's gossmap capacity
  • forward direction: both shrink by imp->amount, saturating at 0

Since #9150, get_constraints() seeds *max from gossmap_chan_get_capacity() instead of -1ULL, so reverse-direction impressions can push max above the channel's real capacity, and enough forward-direction volume drives max to 0 and makes askrene treat a live channel as dead.

The FIXME added to test_xpay_fake_channeld in that PR documents the symptom:

/* FIXME: We fail on #10, due mainly to a buildup of usage on 0x2134x0/0:
 * Failed: We could not find a usable set of paths. The shortest path is
 * 103x1x0->0x2134x0->1725x11x1725, but 0x2134x0/0 exceeds htlc_maximum_msat ~1000448msat
 */

Bounded in practice by two things: mcf.c defensively does if (min > max) min = max, and xpay calls askrene-age with a one-hour cutoff before every payment, so impressions expire. But the layer state is still incoherent in the meantime, and explain_failure.c documents an invariant (total >= max_capacity_known >= known_usable) that impressions can violate, since max can exceed cap_msat.

Minimum fix: clamp max to the channel capacity and min to max inside layer_apply_constraints().

Broader question, raised by Lagrang3 during review: the magnitude by which impressions move the bars is too aggressive. A successful payment proves liquidity of at least amount existed, so subtracting the full amount from max discards information rather than adding it. Worth revisiting the model alongside the clamp.

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