cmd: accept N+ syntax in seg rm-state --step flag

[awskii]

What

erigon seg rm-state --step now accepts an N+ form meaning "from step N to the latest available step":

• --step 5+ — remove everything from step 5 to the latest
• --step 5-10 — unchanged

Previously the only way to express "from step N to the end" was an arbitrarily large upper bound like --step 5-9999999.

How

The --step parsing is extracted into parseStepRange(stepRange, maxAvailableStep). For the N+ form it parses the prefix as from and sets to to the highest toStep among the state files already scanned by DeleteStateSnapshots (respecting any --domain filter). The from-to form is unchanged. Only the range parsing changed — the file-removal logic is untouched.

Tests

• Test_parseStepRange — table-driven unit test covering both formats and error cases (empty, + only, bad prefix, non-numeric).
• Test_DeleteStateSnaps_StepRange_FromPlus — end-to-end: 192+ removes the 192-224 merged file plus its sub-ranges and keeps 0-128 / 128-192, identical to 192-224 since 224 is the highest step.

make lint clean; go test ./cmd/utils/app/ passing.

[awskii]

@copilot review this PR

[Copilot]

@copilot review this PR

Reviewed and pushed 286ff95 to reject malformed ranges like 5-10x and 5-10-20; go test ./cmd/utils/app and make lint are clean.