cli/sdk: `burn export ledger --format jsonl`

[willwashburn]

Parent: #240
Split out from: #248 (scope update originally proposed in #259)

Context

burn export ledger --format jsonl was added to the CLI scope in the #259-driven scope update on #248 but never landed. Neither the SDK verb nor the CLI presenter exists today:

• No export_ledger / equivalent symbol in crates/relayburn-sdk/src/lib.rs.
• No Export variant in crates/relayburn-cli/src/cli.rs::Command.

Scope

SDK (relayburn-sdk)

• New verb (e.g. relayburn_sdk::export_ledger) that streams the canonical event stream out of burn.sqlite as JSONL records.
• Caller-supplied Write sink (so the CLI can stream straight to stdout / a file without buffering the whole ledger in memory).
• Stable record shape — document it as the cross-version interchange format.

CLI (relayburn-cli)

• New burn export ledger --format jsonl [--out -] subcommand.
• --out - (default) streams to stdout; --out <path> writes to a file.
• Honors global --ledger-path.

Acceptance

• relayburn_sdk::export_ledger produces JSONL with one event per line against the cli-golden fixture.
• burn export ledger --format jsonl golden-output test passes (--out - and --out <path>).
• Output is streaming — large fixture ledger does not load fully into memory before the first line is emitted.
• Record shape is documented (README or a doc-comment on the SDK verb) as a stable interchange format.