Add schemas for the custom-constraint tables

[nick-gorman]

Adds validation schemas for the custom-constraint tables, updates unique row validation logic, and timeslice NaN logic.

src/ispypsa/validation/schemas/
  custom_constraints.yaml                → constraint senses (<=, >=, =)
  custom_constraints_lhs.yaml            → LHS terms: term types (incl. load), coefficients
  custom_constraints_rhs.yaml            → RHS limits: timeslice + date_from semantics
  network_expansion_options.yaml         → fix expansion_id cross-ref (constraints_rhs → custom_constraints_rhs)
  network_transmission_path_limits.yaml  → unique: key; NaN-timeslice fallback semantics

Broader changes than just adding the custom constraint schemas

1. Uniqueness is now plain unique:, replacing bespoke custom_validation.
The three tables whose uniqueness key includes a nullable column —

• custom_constraints_lhs → [constraint_id, term_type, variable_name, date_from]
• custom_constraints_rhs → [constraint_id, timeslice, date_from]
• network_transmission_path_limits → [path_id, direction, timeslice]

— previously carried hand-written custom_validation rules whose only job was
to spell out the NaN handling. They are now plain unique: declarations. This
relies on a decision to have unique: treat NaN as equal to NaN (pandas
duplicated() semantics), so at most one otherwise-identical NaN row is
permitted per group — standard SQL UNIQUE would treat NULLs as distinct and
let duplicates through. The contract is recorded in #122, which
also tracks where to document it centrally.

2. A NaN timeslice now means "fallback", not "all conditions".
Previously a no-timeslice row in custom_constraints_rhs and
network_transmission_path_limits was read as a static limit applying to all
demand conditions, effectively mutually exclusive with per-timeslice rows. It
is now a fallback: it applies only at snapshots not covered by any
per-timeslice row for the same group, with per-timeslice rows taking precedence
where they overlap — so the two forms may co-exist. Documented under the
timeslice column in both tables.

This second change is a behavioural contract for whatever applies these limits
downstream: the fallback/precedence logic still needs to be implemented there.

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