fix: bump declared minimum macOS deployment target to 10.15

[tsushanth]

Checklist

• All unit tests pass (no test changes; this only touches `Package.swift` + CHANGELOG).
• All UI tests pass.
• Demo project builds and runs on iOS (no SDK code touched).
• Demo project builds and runs on Mac Catalyst (don't have a Catalyst-capable setup locally — flagging for reviewer; this is precisely the configuration the fix unblocks).
• Demo project builds and runs on visionOS.
• I added/updated tests or detailed why my change isn't tested (Package.swift platform declaration — no runtime behaviour to test).
• I added an entry to the `CHANGELOG.md`.
• I have run `swiftlint` in the main directory and fixed any issues (no Swift source changed).
• I have updated the SDK documentation as well as the online docs (no public API changes).
• I have reviewed the contributing guide.

Why

Closes #317.

`Package.swift` declared `.macOS(.v10_12)` but every macOS-relevant file in the SDK uses Combine, async/await, `Task`, or `UnsafeContinuation` — all of which require macOS 10.15+. There are 651 references to those features across `Sources/SuperwallKit/`.

The 10.12 declaration was never realistic. The issue reporter (and anyone else integrating Superwall into a Mac Catalyst target) hit it as a wall of "is only available in macOS 10.15 or newer" errors deep in Superscript and SuperwallKit source files.

What

Single-character bump in `Package.swift` (`v10_12` → `v10_15`) plus a CHANGELOG entry framed as a fix (deployment-target diagnostics now surface honestly at resolve time instead of failing late in compile).

Podspec is unaffected — `SuperwallKit.podspec` only declares iOS.

Scope

Doesn't change `.iOS(.v13)` or `.watchOS("6.2")`. Doesn't bump SDK version (deliberately tagged `Unreleased` in CHANGELOG — let you decide whether this rides in the next patch or a minor).

Greptile Summary

Bumps the declared minimum macOS deployment target in Package.swift from 10.12 to 10.15, aligning the package manifest with what the SDK's source code actually requires (Combine, async/await, Task, UnsafeContinuation). A matching CHANGELOG entry is added under Unreleased.

• Package.swift: single character change — .macOS(.v10_12) → .macOS(.v10_15).
• CHANGELOG.md: new Unreleased / Fixes bullet explaining the change and the Mac Catalyst integration problem it resolves.

Confidence Score: 5/5

Safe to merge — the change is a one-line manifest correction that makes the declared platform requirement honest; no runtime behaviour is altered.

The only touched files are Package.swift (one character changed in a platform version string) and CHANGELOG.md. The new value 10.15 is the correct minimum for Combine and Swift Concurrency back-deployment, matching what the SDK source already requires. No Swift source, no tests, no public API, and no podspec are affected.

No files require special attention.

Important Files Changed

Filename	Overview
Package.swift	Single-line bump of .macOS(.v10_12) to .macOS(.v10_15), correctly aligning the declared minimum with the actual Combine/Swift Concurrency requirements of the SDK source.
CHANGELOG.md	Adds an "Unreleased / Fixes" entry describing the deployment-target bump; content is accurate and consistent with existing changelog style.

Flowchart

%%{init: {'theme': 'neutral'}}%%
flowchart TD
    A[Integrator adds SuperwallKit\nto Mac Catalyst target] --> B{SPM resolves\nplatform constraints}
    B -- Before PR\n.macOS(.v10_12) --> C[Resolution succeeds\nwith wrong minimum]
    C --> D[Compiler hits Combine /\nasync-await unavailable errors\ndeep in SDK source]
    D --> E[❌ Confusing build failure\nat compile time]
    B -- After PR\n.macOS(.v10_15) --> F[Resolution correctly requires\nmacOS 10.15+]
    F --> G{Host target ≥ 10.15?}
    G -- Yes --> H[✅ Builds successfully]
    G -- No --> I[❌ Clear deployment-target\nerror at resolve time]

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