text_field: secure: true for masked password input

[C-Sinclair]

Summary

Adds a secure: true prop to text_field. iOS renders the field as a SwiftUI SecureField (masked input) instead of TextField. Cleartext still reaches the BEAM via on_change — apps hash/store the value as normal.

%{type: :text_field, props: %{
  value: assigns.password,
  placeholder: "Password",
  secure: true,
  on_change: {self(), :password_changed}
}}

The prop flows through Mob.Renderer.prepare_props/4's generic pass-through; no renderer changes were needed. Native side: MobNode.isSecure BOOL, parsed in mob_nif.m, branched on in MobRootView.swift via a @ViewBuilder field switch.

Version / CHANGELOG

CHANGELOG entry under ## [Unreleased]; no mix.exs bump. Matches the repo's pattern of feature-commits-without-bumps and a separate ride-along Bump to X.Y.Z commit at release-cut time (e.g. b974398).

Android

This PR is iOS-only. Mob's library doesn't own Android Compose UI — that lives per-app, generated from mob_new's MobBridge.kt.eex template. A companion PR against GenericJam/mob_new adds PasswordVisualTransformation + KeyboardType.Password to the template.

Until that ships, secure: true is a graceful no-op on Android: the prop travels through the renderer but the Compose TextField ignores it. No breakage for unpatched apps.

Why no reveal-toggle ("eye" button) yet

Worth its own PR. The iOS implementation needs ZStack-and-opacity to retain focus across SecureField/TextField swaps — the current @ViewBuilder if/else (and any naive toggle) tears the view down on every flip, dropping focus and dismissing the keyboard. Pairs with a trailingIcon slot in Compose. Out of scope here.

Test plan

• mix test — 794 passed (27 doctests, 767 tests), 43 excluded. Two new renderer tests cover secure: true serialising and absence-by-default.
• mix format --check-formatted — clean
• mix credo --strict — clean (949 mods/funs, 0 issues)
• mix compile --warnings-as-errors — clean
• mix erlfmt --check src/ — clean
• xcrun clang-format --dry-run -Werror ios/*.m ios/*.h — clean
• Verified on iOS sim (iPhone 17, iOS 26.2): SecureField renders dots; BEAM :password assign holds cleartext ("hunter2"); Length: 7 label updates live.
• Verified Android contract end-to-end with the matching MobBridge.kt patch applied locally: dots render, cleartext reaches BEAM, length label updates. (Template patch lands in the mob_new companion PR.)
• swiftlint ios/ — not installed locally; the Swift diff is mechanical (single expression swapped for an @ViewBuilder computed property).

Files

• ios/MobNode.h — @property(nonatomic, assign) BOOL isSecure
• ios/mob_nif.m — parses props[@"secure"] alongside keyboard / return_key
• ios/MobRootView.swift — @ViewBuilder var field picks SecureField or TextField; existing modifier chain applies uniformly to both
• test/mob/renderer_test.exs — pass-through and default-absent tests
• CHANGELOG.md — entry under ## [Unreleased]

[GenericJam]

Thanks @C-Sinclair — landed as 290254b in mob 0.6.15 with your authorship preserved. Paired with mob_new#6 (Android Compose side) landed as 4a41f47 in mob_new 0.3.8, so secure: true now works on both platforms in the same release.

CHANGELOG entries merged into the new [0.6.15] block (and mob_new [0.3.8]). Closing since the commit is on master.