feat: add PropertyValue tagged type and form encoder

[t-unit]

Summary

Adds a tagged runtime value type and a form encoder over it in tonik_util. This is additive and ships dark — nothing calls the new code yet, so there is no wire behavior change.

• PropertyValue — a sealed type with two variants, PropertyValue.scalar(String) and PropertyValue.array(List<String>), each holding raw (unescaped) strings. The tag lets the map distinguish a scalar from a single-element array (scalar('x') and array(['x']) both flatten to ['x'] under a plain Map<String, List<String>>) and separates a scalar empty string (key=) from an omitted exploded empty array from a non-exploded empty array — distinctions the flat map cannot carry.
• Form encoder — an extension Map<String, PropertyValue>.toForm(...) that performs all object form percent-encoding in one place via String.uriEncode. It consults per-property explode (from FormFieldEncoding) only when the winning value is an array; a descriptor-less array comma-joins, preserving today's wire output. The encoder owns all form value-emptiness checks (EmptyValueException for an empty map, empty scalar, or empty array when allowEmpty is false).

Behavior

• Object explode: true → one entry per property; an exploded array yields repeated keys, an empty exploded array is omitted, and [''] yields a single empty-value entry.
• Object explode: false → a single collapsed k1,v1,k2,v2 entry, byte-identical to the existing map form encoder at allowReserved: false; fieldEncodings is ignored in this mode.
• Per-property allowReserved applies to values only; keys are always component-encoded. A present per-property descriptor is the complete source of truth for that property, so its allowReserved (non-nullable, default false) takes precedence over the object-level flag.

Tests

Unit tests in tonik_util cover repeated-key explode, the three-way empty-array distinction, comma-in-element encoding with and without allowReserved, the collapse byte-parity case, per-property overrides and precedence, useQueryComponent space rendering, and all three EmptyValueException cases plus their non-throwing counterparts. Expected wire values are literal strings. 100% patch coverage.

Design notes

PropertyValue deliberately omits ==/hashCode and a defensive list copy, following the in-package TonikResult sealed-ADT precedent: it is consumed only by an exhaustive switch and is never compared as an instance — tests assert on the resulting ParameterEntry records, which have value equality. This differs from the sibling FormFieldEncoding, which implements equality because it is a descriptor looked up by key and compared. Both can be revisited if a future consumer starts comparing or retaining these values.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.76%. Comparing base (354c998) to head (35475fe).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #196      +/-   ##
==========================================
+ Coverage   92.74%   92.76%   +0.01%     
==========================================
  Files         174      176       +2     
  Lines       17765    17808      +43     
==========================================
+ Hits        16476    16519      +43     
  Misses       1289     1289              

Files with missing lines	Coverage Δ
...nik_util/lib/src/encoding/form_field_encoding.dart	80.00% <ø> (ø)
...es/tonik_util/lib/src/encoding/property_value.dart	100.00% <100.00%> (ø)
.../lib/src/encoding/property_value_form_encoder.dart	100.00% <100.00%> (ø)

Files with missing lines	Coverage Δ
...nik_util/lib/src/encoding/form_field_encoding.dart	80.00% <ø> (ø)
...es/tonik_util/lib/src/encoding/property_value.dart	100.00% <100.00%> (ø)
.../lib/src/encoding/property_value_form_encoder.dart	100.00% <100.00%> (ø)

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