Add exact units, signed time, and interval type-system core

[isomorphisms]

Scope

Small tests-first implementation of the algebraically settled slice from #28, #30, and #31.

Exact quantities / dimensions

• exact rational fixture scalar;
• dimension exponents for Length/Mass/Time;
• multiplication/division normalize compound dimensions;
• exact metre/cm/mm/in/ft conversions;
• litre and explicitly named US gallon/quart/pint/cup scales;
• gallon / second typechecks as Volume / Time without a special compound-unit class.

Time

• Instant kept distinct from signed Duration;
• add/subtract/negate duration;
• instant ± duration;
• instant difference;
• exact seconds/minutes/hours only;
• no calendar/timezone/DST semantics.

No Group/Semigroup inheritance hierarchy is added. The algebraic classification can remain knowledge available to later computer-science/AICI reasoning while ordinary source only sees the operations it needs.

Intervals

• exact rational one-dimensional intervals;
• closed/open/half-open endpoints;
• empty interval handling for reversed bounds and open singleton bounds;
• membership;
• interval addition;
• endpoint openness uses typed +epsilon / -epsilon boundary vocabulary;
• nilpotent dual-number epsilon (epsilon^2 = 0) remains a separate type and is deliberately not ordered;
• no fake inverse that "adds epsilon back" and silently recloses an endpoint.

Deliberately deferred: interval multiplication/division, outward floating rounding, correlated uncertainty (x-x dependency), probability/Gaussian inference, calendar time, and the broader open-ended uncertainty design.

Tests-first history

• tests first: ae6bcfbf0e7ccae46f57a171be40b0c78fc73ab9
• implementation: 57de2109ff81b4f4bf7f2610a63d34b5278b3804
• real compiler harness: edric007
• executable-bit correction: a3ffc1c03e040e31a0867d473611b1bf8960cca0

Prior exact-head verification

The mathematical slice was VERIFIED on feature head a3ffc1c03e040e31a0867d473611b1bf8960cca0 in its then-current pull-request merge context.

• tested PR merge commit: 59ad1d9c83028ca1dbdb8ba50f486cdd7dca9425
• workflow: Edric type-system smoke
• run: 33277502908
• job: 99166698046
• full ./edric all bootstrap completed successfully
• focused receipt: idris2/basic/edric007: success 00.854s
• output comparison: 1/1 tests successful

The only correction after the first semantic green run was making edric007/run executable; no mathematical/type semantics were weakened or changed.

Reconciliation onto current Idriç

Current head: 7b7df7638aa7e464a11388da1add72d06d6a977a

Current base incorporated: 414cefdb9b96ec71f798559f23a50fb642774597

The reconciliation is a two-parent merge preserving the verified head in history. It adapts only the repository-layout move:

• relocates the example to _/examples/units-time-intervals/;
• relocates edric007 to _/tests/idris2/basic/edric007/;
• adds edric007 to the current _/edric smoke list;
• updates the existing type-smoke workflow paths and entrypoint to ./_/edric all.

Tests.idric, UnitsTimeIntervals.idric, edric007/expected, and edric007/run are carried forward byte-for-byte from a3ffc1c03e040e31a0867d473611b1bf8960cca0, including the executable mode on edric007/run.

Current acceptance boundary

The prior green receipt proves the unchanged mathematical slice on its exact old head. It is not treated as proof that the new base integration is green. Before merge, the same Edric type-system smoke / units-time-intervals check must pass on current head 7b7df7638aa7e464a11388da1add72d06d6a977a.

No deferred interval, uncertainty, probability, calendar/time-zone, floating-rounding, or algebraic-hierarchy work is pulled into this reconciliation.

Related: #28, #30, #31, #42.

[isomorphisms]

Self-check boundary: this PR intentionally implements only the exact rational/unit, signed-duration/instant, and additive interval kernels. Do not broaden it to calendar semantics, probability distributions, general uncertainty, or algebraic interface inheritance merely to make the first slice feel complete. Compiler failures should be repaired minimally without weakening the pinned equations.

[isomorphisms]

Cross-reference for later review: #28 owns dimensions/units, #30 owns intervals/uncertainty, #31 owns instants/durations/calendar separation, and #42 owns broader constraint inference. This PR is the common executable first slice, not a replacement for those design issues.