AssumeUTXO snapshot 201500 (replaces 199300)

AssumeUTXO snapshot at height 201500

Replaces assumeutxo-199300, whose base ff80e629 sits on the withdrawn 0.34.1 branch
(issue #127). That snapshot stranded any node that used it: it starts on an abandoned
branch and cannot converge, and because 0.34.3 correctly refuses reorgs that would
disconnect a snapshot base, invalidateblock cannot recover it either. Do not use
assumeutxo-199300. It is superseded by this one.

Why this base is trustworthy

It was dumped from a node that had demonstrably converged with the live network — it
synced from 199298 to the network tip on a stock config with no manual intervention, and
was at 201620 when this snapshot was taken. The base at 201500 is therefore an ancestor of
a tip that the network agrees on, rather than a height we asserted was correct.

Independently corroborated: @Jpp-matata's integer-exact ASERT recomputation across five
branches found the majority branch 199,296→200,222 clean over 927 transitions (zero nBits
mismatches, zero timestamp-rule violations), while the withdrawn 0.34.1 branch fails at
199,303 (0x1e2ba2c4 vs expected 0x1e2b8596).

Contents

base_height                201500
base_hash                  3dd0fa677029f0b6869b64f09d8673edf3902460767bd6a1ecf6c633b0c6398c
txoutset_hash              4743962b836a3ed1e541bb6da747fc28a7d98926b5d6bc8e23928ed3b1981d93
nchaintx                   301211
coins_written              144406
shielded_state_pin         94343b766b39c0ea2d92d83323f77b5ccc5e775d99b34b01f5fa6400f2354541
shielded_retention_profile full_commitment_index
sha256                     08c52c8b34e878c4d48546cfec066bc48fceed51d7287b4ff7ec7b5727cf52c7

The shielded pin matches the commitment carried by the highest remaining compiled snapshot
(191266, 94343b76…) after the 199300 pin was withdrawn.

Use

sha256sum -c SHA256SUMS-assumeutxo-201500.txt
btx-cli loadtxoutset /path/to/btx-assumeutxo-201500.dat

Requires a build whose chainparams carries the 201500 entry. Verify the sha256 before
loading — that is the point of publishing it.

Recovery path

This also restores the recovery route for #74 (an unclean shutdown forcing a shielded-state
rebuild), which had no snapshot to fall back on once the 199300 pin was withdrawn.

Note: nodes that already bootstrapped from assumeutxo-199300 are on the withdrawn branch
and cannot be rescued in place. Those need a resync from an empty datadir.