feat: count exclusive candidates dropped for invalid route shape

[tamaralipows]

An exclusive leg that does not reach the route's output token makes the candidate unusable, so no commitment is made and the order falls back to public liquidity. That happened silently.

exclusive_route_invalid_shape_total counts it:

if !reaches_output {
    counter!("exclusive_route_invalid_shape_total").increment(1);
    return false;
}

It counts only the wrong-position case, not routes that carry no exclusive leg at all — those are pools routing around the exclusive components, not rejections, and they would swamp the series.

The counter fires per exclusive-scope candidate, not per order. An order served by several exclusive-scope pools contributes several increments.

🤖 Generated with Claude Code

[tamaralipows]

Reopening. The shape rule is still on main, so this is the number that sizes whether removing it is worth doing.

#446 sketched the removal and was closed on cost: the conversion is a bisection over route replays, and a vm:* pool downstream of the exclusive leg makes those replays expensive against the 1000 ms router budget. non_terminal_exclusive_leg here says how often that case actually arises, which decides whether the cheaper version is worth building.

[github-actions]

Breaking API Changes Detected

This PR introduces breaking API changes, but the PR title does not declare them.
Either fix the breaking changes, or declare them:

• Append ! to the PR title type (e.g. feat!: ...) and make sure a !-marked commit
  lands on main — release-plz bumps the minor version for breaking changes on 0.x.x.

semver-checks output

error: failed to load rustdoc from file at `"target/doc/fynd_core.json"`

Caused by:
    unsupported rustdoc format v60 for file: target/doc/fynd_core.json
    (supported formats are v55, v56, v57)
error: failed to load rustdoc from file at `"target/doc/fynd_test_fixtures.json"`

Caused by:
    unsupported rustdoc format v60 for file: target/doc/fynd_test_fixtures.json
    (supported formats are v55, v56, v57)
error: failed to load rustdoc from file at `"target/doc/fynd_rpc.json"`

Caused by:
    unsupported rustdoc format v60 for file: target/doc/fynd_rpc.json
    (supported formats are v55, v56, v57)
error: failed to load rustdoc from file at `"target/doc/fynd_rpc_types.json"`

Caused by:
    unsupported rustdoc format v60 for file: target/doc/fynd_rpc_types.json
    (supported formats are v55, v56, v57)
error: failed to load rustdoc from file at `"target/doc/fynd_client.json"`

Caused by:
    unsupported rustdoc format v60 for file: target/doc/fynd_client.json
    (supported formats are v55, v56, v57)
error: failed to load rustdoc from file at `"target/doc/fynd_tools_common.json"`

Caused by:
    unsupported rustdoc format v60 for file: target/doc/fynd_tools_common.json
    (supported formats are v55, v56, v57)
error: failed to load rustdoc from file at `"target/doc/erc20_overrides.json"`

Caused by:
    unsupported rustdoc format v60 for file: target/doc/erc20_overrides.json
    (supported formats are v55, v56, v57)