feat: Add SentrySPM compile-from-source product and multi-platform sample apps

[philprime]

Summary

• Adds a new SentrySPM SPM product that compiles Sentry directly from source via a thin Swift wrapper module (Sources/SentrySPM/) using @_exported import, so consumers only need import SentrySPM
• Restructures the compile-from-source targets: renames SentryObjc to SentryObjCInternal (reserving SentryObjC for a future public wrapper), adds extensive inline documentation in Package.swift
• Adds SwiftUI sample apps for all supported platforms (iOS, watchOS, tvOS, macOS, visionOS) using SentrySPM with XcodeGen specs, Makefile build targets, and CI workflow integration

Architecture

SentryHeaders      → public ObjC headers (Sources/Sentry/Public/)
_SentryPrivate     → internal ObjC headers (Sources/Sentry/include/)
SentrySwift        → all Swift source (Sources/Swift/)
SentryObjCInternal → all ObjC/C source (Sources/Sentry/, Sources/SentryCrash/)
SentrySPM          → thin re-export wrapper (@_exported import of ObjC + Swift)

Test plan

• CI builds all five platform sample apps (iOS, watchOS, tvOS, macOS, visionOS) via build-sample-spm matrix job
• Verify make build-sample-iOS-SwiftUI-SPM succeeds locally
• Verify existing SPM/xcframework products are unaffected
• Verify import SentrySPM provides access to both ObjC types (SentrySDK, Breadcrumb) and Swift types (.sentryTrace())

#skip-changelog

Closes #7431

[github-actions]

Semver Impact of This PR

🟡 Minor (new features)

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[github-actions]

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📖	Do not forget to update Sentry-docs with your feature once the pull request gets approved.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85.817%. Comparing base (92e1256) to head (b4748ff).
⚠️ Report is 8 commits behind head on main.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@              Coverage Diff              @@
##              main     #7430       +/-   ##
=============================================
+ Coverage   85.318%   85.817%   +0.498%     
=============================================
  Files          480       480               
  Lines        28615     28620        +5     
  Branches     12385     12391        +6     
=============================================
+ Hits         24414     24561      +147     
+ Misses        4155      4010      -145     
- Partials        46        49        +3     

see 16 files with indirect coverage changes

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[philprime]

See techprimate/Flinky#92 for additional real-world sample

[github-actions]

Performance metrics 🚀

	Plain	With Sentry	Diff
Startup time	1188.40 ms	1210.33 ms	21.94 ms
Size	24.14 KiB	1.11 MiB	1.09 MiB

Baseline results on branch: main

Startup times

Revision	Plain	With Sentry	Diff
59981b9	1207.25 ms	1240.71 ms	33.46 ms
0d145c3	1224.04 ms	1255.34 ms	31.30 ms
17b7e88	1223.04 ms	1253.48 ms	30.43 ms
79e2bb8	1216.37 ms	1242.42 ms	26.05 ms
79cee27	1214.65 ms	1238.66 ms	24.01 ms
9a34176	1227.24 ms	1266.22 ms	38.98 ms
2400b80	1230.26 ms	1261.19 ms	30.93 ms
fd24b7c	1222.39 ms	1259.67 ms	37.28 ms
4402b70	1214.75 ms	1252.13 ms	37.38 ms
a424cf3	1231.36 ms	1259.62 ms	28.25 ms

App size

Revision	Plain	With Sentry	Diff
59981b9	24.14 KiB	1.09 MiB	1.06 MiB
0d145c3	24.14 KiB	1.07 MiB	1.05 MiB
17b7e88	24.14 KiB	1.06 MiB	1.04 MiB
79e2bb8	24.14 KiB	1.04 MiB	1.02 MiB
79cee27	24.14 KiB	1.10 MiB	1.08 MiB
9a34176	24.14 KiB	1.10 MiB	1.07 MiB
2400b80	24.14 KiB	1.10 MiB	1.07 MiB
fd24b7c	24.14 KiB	1.07 MiB	1.04 MiB
4402b70	24.15 KiB	1.10 MiB	1.08 MiB
a424cf3	24.14 KiB	1.06 MiB	1.04 MiB

Previous results on branch: philprime/spm-sample

Startup times

Revision	Plain	With Sentry	Diff
becdd0e	1228.96 ms	1257.13 ms	28.17 ms
4a23cfb	1226.64 ms	1252.13 ms	25.50 ms

App size

Revision	Plain	With Sentry	Diff
becdd0e	24.14 KiB	1.11 MiB	1.09 MiB
4a23cfb	24.14 KiB	1.11 MiB	1.09 MiB

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