Deprecate Shared Instant and replace uses of it with Kotlin Instant

[stevenzeck]

Since Kotlin 2.1.20, Instant has been available in the Kotlin standard library. This PR does the following:

• Adds a helper extension function, String.toInstant(), to parse strings into Kotlin Instant
• Replaces the use of the custom Instant in the Shared module throughout the toolkit, using toInstant where necessary.
• Deprecates said custom Instant and its functions, and documents and suggests replacements.

[Copilot]

Pull request overview

This PR migrates the toolkit away from the custom org.readium.r2.shared.util.Instant wrapper by adopting the Kotlin standard library kotlin.time.Instant, adding a shared parsing helper (String.toInstant()), updating call sites, and deprecating the legacy wrapper API.

Changes:

• Added String.toInstant() to parse ISO-8601-like date strings (with fallback parsing) into Kotlin instants.
• Replaced usages of the legacy Shared Instant across Shared/OPDS/Streamer/LCP/Test app modules with kotlin.time.Instant and epoch-millis conversions where needed.
• Deprecated the legacy Instant wrapper and documented migration guidance in the changelog.

Reviewed changes

Copilot reviewed 26 out of 26 changed files in this pull request and generated 6 comments.

Show a summary per file

File	Description
test-app/src/main/java/org/readium/r2/testapp/reader/PublicationMetadataDialogFragment.kt	Switches publication date handling to kotlin.time.Clock/System.now() and epoch-millis to java.util.Date.
test-app/src/main/java/org/readium/r2/testapp/drm/LcpManagementViewModel.kt	Replaces legacy toJavaDate() with Date(instant.toEpochMilliseconds()) conversions.
readium/streamer/src/test/java/org/readium/r2/streamer/parser/epub/MetadataTest.kt	Updates imports after Instant migration (currently introduces an unused import).
readium/streamer/src/main/java/org/readium/r2/streamer/parser/epub/MetadataAdapter.kt	Uses the new String.toInstant() when parsing EPUB metadata dates.
readium/shared/src/test/java/org/readium/r2/shared/util/InstantTest.kt	Suppresses deprecation warnings for tests that still cover the legacy wrapper.
readium/shared/src/test/java/org/readium/r2/shared/publication/MetadataTest.kt	Migrates test data from Instant.parse(...) to String.toInstant().
readium/shared/src/test/java/org/readium/r2/shared/opds/AvailabilityTest.kt	Migrates test data from Instant.parse(...) to String.toInstant().
readium/shared/src/main/java/org/readium/r2/shared/util/Instant.kt	Deprecates the legacy Instant wrapper and suggests replacements.
readium/shared/src/main/java/org/readium/r2/shared/publication/Metadata.kt	Switches modified/published to Kotlin Instant and parses via toInstant().
readium/shared/src/main/java/org/readium/r2/shared/opds/OpdsMetadata.kt	Switches OPDS metadata modified to Kotlin Instant.
readium/shared/src/main/java/org/readium/r2/shared/opds/Availability.kt	Switches OPDS availability instants and parsing to Kotlin Instant/toInstant().
readium/shared/src/main/java/org/readium/r2/shared/extensions/String.kt	Introduces String.toInstant() extension for permissive parsing.
readium/opds/src/test/java/org/readium/r2/opds/OPDS1ParserTest.kt	Updates OPDS1 parser tests to use String.toInstant() for dates.
readium/opds/src/main/java/org/readium/r2/opds/OPDS2Parser.kt	Updates OPDS2 parsing to use String.toInstant() for modified.
readium/opds/src/main/java/org/readium/r2/opds/OPDS1Parser.kt	Updates OPDS1 parsing to use String.toInstant() for modified/published.
readium/lcp/src/main/java/org/readium/r2/lcp/license/model/components/lsd/PotentialRights.kt	Switches LSD rights end date parsing to Kotlin Instant via toInstant().
readium/lcp/src/main/java/org/readium/r2/lcp/license/model/components/lsd/Event.kt	Switches event timestamp parsing to Kotlin Instant via toInstant().
readium/lcp/src/main/java/org/readium/r2/lcp/license/model/components/lcp/Rights.kt	Switches start/end parsing to Kotlin Instant via toInstant().
readium/lcp/src/main/java/org/readium/r2/lcp/license/model/StatusDocument.kt	Switches updated timestamps parsing to Kotlin Instant via toInstant().
readium/lcp/src/main/java/org/readium/r2/lcp/license/model/LicenseDocument.kt	Switches issued/updated parsing to Kotlin Instant via toInstant().
readium/lcp/src/main/java/org/readium/r2/lcp/license/LicenseValidation.kt	Switches “now” to Clock.System.now() for license validation logic.
readium/lcp/src/main/java/org/readium/r2/lcp/license/License.kt	Migrates license API surface to Kotlin Instant.
readium/lcp/src/main/java/org/readium/r2/lcp/MaterialRenewListener.kt	Uses Clock.System.now() and epoch millis for the date picker integration.
readium/lcp/src/main/java/org/readium/r2/lcp/LcpLicense.kt	Migrates LCP license API to Kotlin Instant.
readium/lcp/src/main/java/org/readium/r2/lcp/LcpError.kt	Migrates error model references to Kotlin Instant.
CHANGELOG.md	Documents the deprecation and migration guidance for instants.

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[stevenzeck]

LocalDateTime.toInstant and LocalDate.atStartOfDayIn both return kotlin.time.Instant.

[stevenzeck]

Same comment as String.kt.