feat: support virtual attribute defaults

[elpete]

Closes #113

Review

This improvement is a good fit for Quick. Virtual projections often represent counts or booleans, and a new entity needs a meaningful value before a query has populated the projection.

Recommendation: 9/10.

Reasons for

• Avoids an unnecessary database refresh after saving a newly constructed entity.
• Makes virtual attributes behave more like declared properties with defaults.
• Ensures API mementos have stable types such as false instead of an empty string.
• Carries the default through newEntity() metadata reuse.

Reasons against

• This intentionally changes the positional meaning of the second argument.
• Callers that currently use appendVirtualAttribute( name, true ) to exclude an attribute must move that flag to the third argument or use the named excludeFromMemento argument.
• Dynamically mutating an entity's memento includes requires care when custom memento configuration is present.

Implementation

• Changes the signature to appendVirtualAttribute( name, defaultValue, excludeFromMemento = false ) on both BaseEntity and QuickBuilder.
• Stores the default in virtual-attribute metadata and initializes it on the current entity and entities produced by newEntity().
• Adds a dynamically appended virtual attribute to default memento includes unless explicitly excluded.
• Preserves named internal excludeFromMemento calls.

Test-first evidence

Before implementation, the public magic getter returned an empty string for appendVirtualAttribute( "hasPosts", false ); the new test failed with Expected [false] but received [""].

Tests now verify:

• a false default through the generated/magic getter;
• the default in the normal memento flow;
• propagation through newEntity(); and
• the third-position memento exclusion flag.

Validation

• Attribute, memento, and subquery specs: 35 passed, 0 failed, 0 errors
• Full suite: 497 passed, 0 failed, 0 errors, 3 skipped
• Formatter completed
• git diff --check passed
• Tested with qb@14.0.0-beta.3