Convert a value or unit expression from one unit to another (GNU units engine, TODO §16). The universal conversion core: one tool covers every category (length, mass, time, temperature, area, volume, energy, power, speed, data, …) plus compound expressions like `kW*h` or `acre*ft`. Linear conversions return the ratio of coefficients; a nonlinear target (e.g. `tempF`) applies that unit's inverse. Returns: `from` and `to` (echoed), `result` (the converted magnitude WITH the target unit, e.g. "1.609344 km" — the primary human-readable answer), `value` (the same magnitude as a bare float for programmatic use), and `exact` (true when the result is exact, false when it was rounded). Errors cleanly (isError) when a unit is unknown, an expression is malformed, or the two sides are not conformable. Example: convert("1 mile", "km") -> {"from":"1 mile","to":"km","result":"1.609344 km","value":1.609344,"exact":true}

Reduce a value or unit expression to SI base units. Reduces any linear unit to its SI base-unit (primitive) form: `km` → `1000 m`, `hour` → `3600 s`, `kW*hour` → `3.6e+06 kg m^2 / s^2`, etc. Use it to check whether a unit is valid, canonicalize quantities for comparison, or run dimensional analysis (the signature of the reduced form is the quantity's physical dimension). Nonlinear units (e.g. `tempF`, `dB`) have no linear SI form and error. Raise an `isError` result when a unit is unknown, an expression is malformed, or the unit is nonlinear. Example: convert_to_si("55 mile/hour") -> {"expr":"55 mile/hour","result":"24.5872 m / s","value":24.5872,"exact":false, "dimension":"m / s"}

Search the GNU units database for units whose name or definition contains a keyword. Basic substring search over the 3000+ unit GNU units database (TODO §14.1). A hit is any unit whose name OR definition text contains `query` (case-insensitive); prefixes are excluded. Results are returned in the database's native order and capped at `limit`. Use this to discover the exact spelling of a unit before calling a conversion tool. Returns: `query` (echoed), `count` (number of results returned), and `results`, a list of objects each carrying: - `name` : the unit's name. - `definition` : its definition text (the source line with the leading name token removed, so the name is not repeated). - `kind` : "unit" | "primitive" | "function" | "table". - `dimension` : the unit reduced to base-unit signature, e.g. "kg m^2 / s^3" for power — use it to check whether two units are conformable without a second call. Omitted for hits that do not reduce (functions, tables). - `base_value` : the unit reduced to base/primitive units, coefficient included, e.g. "745.699 kg m^2 / s^3". Omitted alongside `dimension` when the hit does not reduce. Enriching each hit lets you search AND triage in a single call instead of a follow-up lookup per unit. Example: find_units("horsepower") -> {"query":"horsepower","count":9,"results":[{"name":"horsepower", "definition":"550 foot pound force / sec","kind":"unit", "dimension":"kg m^2 / s^3","base_value":"745.7 kg m^2 / s^3"}, ...]}