feat(loadtest): per-transaction calldata from stdin for contract-call mode

[praetoriansentry]

Description

--calldata and --calldata-file send the same payload in every contract-call transaction. This adds --calldata-stdin and --calldata-size so each transaction gets its own calldata from a shell pipeline.

• Stdin is read as raw bytes and cut into --calldata-size chunks. Each transaction carries the next chunk. A trailing partial chunk is discarded with a warning.
• The test stops cleanly at EOF or at the request count, whichever comes first. A read error on stdin ends the run non-zero so a dead producer is not mistaken for a clean finish.
• New mode.InputReserver interface. The runner reserves the input before taking a nonce, so an exhausted stream never leaves a reserved nonce unsent (which would stall the final-block wait). Once a request holds a nonce it always runs to completion.
• Validation: requires contract-call as the sole mode; mutually exclusive with --calldata, --calldata-file, and --reverse-nonce-order; rejects a terminal on stdin; caps chunk size at 1 MiB.
• No selector prefix is added. Callers prepend one in the input if the target needs it.

Not breaking. No new dependencies (golang.org/x/term was already direct).

Pre-existing gap noticed while here, not changed: the uniswapv3 subcommand never calls cfg.Validate(). The new flags are local to the parent command so it cannot reach them.

Jira / Linear Tickets

• N/A

Testing

• go test -race ./loadtest/... ./cmd/..., golangci-lint, go vet, typos, go run docutil/*.go all clean.
• New unit tests: validation table (TestValidateCalldataStdin), chunk feeder (exact multiple, partial tail, empty input, read error, cancellation while blocked, reader exit when buffer full, 16 concurrent consumers with -race), contract-call mode (per-chunk calldata via reserve + execute, unreserved execute fails, stale feeder cleared on Init, legacy vs dynamic fee tx build).
• Against local anvil, EOA target, --gas-limit 100000:
  • 25 chunks, --concurrency 8 --requests 100: exactly 25 txs sent, stopped at EOF, final-block wait completed, 0 errors, exit 0.
  • 25 chunks, --concurrency 8 --requests 3: exactly 24 txs sent (request cap wins).
  • --output-raw-tx-only: decoded raw txs carry different input per chunk.
  • 35 bytes at size 30: one tx sent, warning about 5 stray bytes.
  • < /dev/null: 0 txs, clean stop.
  • Pseudo-terminal on stdin (via script): rejected with the intended message.
  • --reverse-nonce-order and --calldata-size 2000000: rejected at validation.

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