Restore configurable insert-method selection

[Cianidos]

Summary

Restores the configurable insert-method selection that buildDriverConfig had been silently discarding. defaultInsertMethod was parsed from presets, raw JSON, config files, and -D, but every workload kept its hard-coded InsertRequest.Method and invalid values succeeded.

What changed

• New typed run parameter --insert-method (plain_query, plain_bulk, columnar, native) with process env INSERT_METHOD, -e INSERT_METHOD, and config run.insertMethod — the highest-precedence user override.
• Driver-level defaultInsertMethod restored: preset/JSON/drivers[N].defaultInsertMethod/-D insertMethod/-D defaultInsertMethod now flows into config.DriverConfig.InsertMethod and is validated at driver-build time via driver.ParseInsertMethod.
• driver.ResolveInsertMethod validates the value and rejects methods the selected driver does not serve — before loading starts.
• Bench.Insert applies the resolved method to every load request (tpcc/tpcb/simple/tpch/tpcds all route through it); a zero value keeps the workload's own method.
• Probe stays authoritative (driver.InsertCapabilities) and help/schema enumerate all values including columnar.

Precedence (highest → lowest)

1. --insert-method (CLI > INSERT_METHOD env > -e > config run.insertMethod)
2. driver-level defaultInsertMethod (-D/JSON > config drivers[N] > preset)
3. workload-authored default

Tests

• driver.ResolveInsertMethod/SupportsInsertMethod/InsertMethods unit tests (valid, invalid, unsupported).
• bench resolution precedence test + a fake driver proving the resolved method reaches Driver.Insert (and that the workload default is kept when unset).
• run.buildDriverConfig resolution tests (valid/invalid/unsupported + -D insertMethod aliases).

Closes #133

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[Cianidos]

Retarget blocked: runtime config design decision required

#152 cannot be mechanically detached from skipped #150/#151 and retargeted to current main.

A dry rebase of only this PR's commits (e4ad1b50..4d9af7e3) onto current main stopped on the first commit with conflicts in AGENTS.md, CHANGELOG.md, cmd/stroppy/commands/run/run.go, run_test.go, plus a modify/delete conflict for #150-only pkg/config/config.go.

Current main has a protobuf DriverRunConfig.defaultInsertMethod, but runtime DriverConfig has no field carrying the resolved method into bench.Run and drivers. This PR assumes #150's plain-Go runtime config and #151's removed environment bridges.

A maintainer must choose among functionally different designs:

1. Extend the frozen protobuf runtime DriverConfig (field number, descriptor/reflection, JSON surface, getter/API).
2. Keep protobuf unchanged and pass the resolved method separately through bench.Run or a new options type.
3. Model it as another parameter-input source, changing source attribution and precedence behavior.

Two further policy choices affect observable behavior:

• Keep preset defaultInsertMethod values, or remove them so bare presets cannot override workload-authored defaults.
• Reject invalid lower-tier driver values even when a higher-precedence typed input wins, or validate only the selected effective value.

Merging this PR as-is updates #151's blocked branch, not main. Retargeting the existing head directly to main includes all seven blocked #150/#151 commits; the unique-only transplant requires the design choices above.

Please choose the runtime transport, preset-default policy, and validation timing, then restack only the selected #152 behavior on current main. Per the sequential merge rule, this PR is skipped; no branch changes were pushed.