Add GetOrAdd and GetOrAddFunc to the single-component handle

[herve-quiroz]

Summary

Adds a first-class get-or-create operation to the single-component handle (Accessor[A]), as specified in #3:

• GetOrAdd(id, value) *A — returns the interior pointer to the entity's component, inserting value first if absent (eager).
• GetOrAddFunc(id, make) *A — same, but builds the value lazily via make, which is called only on the miss path (nothing constructed on a hit).

Both return a non-nil *A, valid until the next structural change to the store (the same contract as Get and iteration).

Design notes

The add honours the existing deferral model exactly as Add does:

• At depth 0 (outside iteration): immediate. A dead entity is not stored (mirroring Add's no-op), but a non-nil pointer to the unstored value is still returned to keep the "never nil" contract.
• During an All() loop: deferred. Because the result must remain a usable pointer for the rest of the loop, the value is staged off-store and the returned pointer points at it; the auto-flush inserts that same value, so writes made through the pointer during the loop survive into the store. No mutation touches the dense storage mid-iteration, so previously-yielded iteration pointers stay valid.

Both methods delegate to a shared private insertMissing helper after the get-miss check, so the eager/lazy split is the only difference between them.

Scope is the single-component handle only; the join handles (Accessor2/Accessor3) are out of scope per the issue.

Testing

New tests in ecs_components_test.go cover: insert-on-miss (both variants), return-existing-without-overwrite, make not called on hit, dead-entity returns non-nil without storing, deferred-during-iteration (mutation through the staged pointer lands after flush), and hit-path-during-iteration returning a live pointer.

Verification (all clean):

gofmt -l ecs
go vet ./...
go build ./...
go test ./... -race

Docs

Updated the README's "Core concepts" and "API at a glance" to list the new get-or-create pair.

fixes #3

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