… dep (erigontech#21197)

Fixes erigontech#21154. Fixes erigontech#19732. Sub-task of erigontech#21047.

## Summary
- Drop the `github.com/erigontech/erigon-snapshot` Go-module import. The
embedded TOMLs it ships were loaded at startup, immediately overwritten
by a runtime fetch, and discarded — they have been unused on every
daemon path since erigontech#12415 made remote-fetch failure fatal.
- Drop the `--preverified=embedded` flag value (a dev convenience from
erigontech#18273); `remote` and `local` remain.
- Clean up the now-vestigial registry pieces: remove the unused
`preverifiedRegistry.Reset` method (dead since erigontech#19641 switched to
per-chain loading), promote the immutable supported-chain set to a
package-level `knownChains` var, and inline its only membership-check
consumer in `SetToml`.

Runtime fetch source
(`raw.githubusercontent.com/erigontech/erigon-snapshot` + R2 mirror) and
fail-fast behaviour are unchanged. Binary size: −2,973,120 bytes
uncompressed (−2.0%) / −1,015,102 bytes gzipped (−1.6%) on
`darwin/arm64`, measured by building before/after a stubbed-empty
`erigon-snapshot`.

## Test plan
- [x] `make lint && make erigon integration` clean
- [x] `go test ./db/snapcfg/... ./db/downloader/downloadercfg/...` pass
- [x] Manual: `--chain=hoodi` with both CDN hosts unreachable
(`HTTPS_PROXY` pointed at a dead local port) exits non-zero with the
same fail-fast `[CRIT] Snapshot hashes for supported networks was not
loaded …` startup trace
- [x] Manual: `--chain=hoodi` with normal network logs `Loading remote
snapshot hashes chain=hoodi`, no `Failed to load` warning, no `[CRIT]`,
and progresses into the downloader (segments begin downloading)
- [x] Manual: `--chain=hoodi` against a fresh datadir prepopulated with
a real `<datadir>/snapshots/preverified.toml` (fetched out-of-band)
starts cleanly with HTTPS_PROXY pointed at a dead local port — no
`Loading remote snapshot hashes` log line, no `[CRIT]`, downloader
brings up — confirming the local-file path bypasses remote fetch
- [x] Manual: `erigon snapshots reset --datadir=<dd>
--preverified=embedded --dry-run` exits 1 with `Error: invalid
preverified flag value "embedded"`; the `--help` output shows `(remote,
local)`; `--preverified=remote` and `--preverified=local` continue to
work