wallet: build RingCT output distribution on short chains (fakechain/regtest)

[Notsosmartt-cmd]

get_rct_distribution floors the per-asset output-distribution query's from_height
at the mainnet fork heights (AUDIT_FORK_HEIGHT for ZPH/ZSD/ZRS/ZYS,
YIELD_FORK_HEIGHT for ZYIELD). On a chain shorter than that floor - a fakechain or
regtest network - from_height exceeds the chain tip, the daemon's
get_output_distribution returns false ("failed to get output distribution"), and no
RingCT transaction can be built (an isolated self-transfer, or any spend, fails).

This clamps from_height to genesis when it is at or past the current blockchain
height: a strict no-op on mainnet (height is far past the fork floors), and on a short
chain it counts from genesis, which already contains enough coinbase-derived amount=0
RingCT outputs to satisfy the ring size. It unblocks RingCT on private chains - which
downstream regtest/CI tooling needs (e.g. atomic-swap test harnesses driving ZEPH on a
private chain) - without changing mainnet behaviour.

The change is one branch in wallet2::get_rct_distribution, right after the existing
per-asset from_height assignment. Verified functionally: the daemon's
get_output_distribution succeeds at from_height=0 on a short regtest chain (and
still fails at AUDIT_FORK_HEIGHT, as before), and ZEPH RingCT spends build on a short
chain after this change.

[Notsosmartt-cmd]

Note: this is a standalone improvement (RingCT on regtest/fakechain for any Zephyr dev or CI tooling), not tied to any specific downstream. Happy to add a test or adjust the framing if that helps review.