Add inline suggestion display API

[M09Ic]

Background

Applications that build interactive shells often have suggestions that do not come from readline history: AI command suggestions, remote completions, or application-specific prediction engines. These suggestions are best displayed as ghost text after the cursor, similar to history autosuggest, but callers currently have no public API to provide such text.

Approach

• Add Shell.SetInlineSuggestion, Shell.ClearInlineSuggestion, and Shell.GetInlineSuggestion.
• Store the suggestion in the display engine and render only the suffix after the current line.
• Apply the suggestion only when the cursor is at the end of the line and the suggestion extends the current line.
• Keep existing history autosuggest behavior first, so history suggestions continue to win when enabled and available.
• Include the inline suggestion in display coordinate calculation so wrapping and cleanup account for the ghost text width.

Tests

• Added public API coverage for set/get/clear.
• Added display-engine tests for prefix matching, cursor-position gating, coordinate line selection, and history-autosuggest precedence.
• Ran go test ./...

[codecov]

Codecov Report

❌ Patch coverage is 57.77778% with 19 lines in your changes missing coverage. Please review.
✅ Project coverage is 38.81%. Comparing base (088046b) to head (6ce8284).

Files with missing lines	Patch %	Lines
internal/display/engine.go	66.66%	9 Missing ⚠️
shell.go	50.00%	3 Missing and 3 partials ⚠️
internal/display/refresh.go	33.33%	3 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #114      +/-   ##
==========================================
+ Coverage   38.33%   38.81%   +0.47%     
==========================================
  Files          59       59              
  Lines        9516     9556      +40     
==========================================
+ Hits         3648     3709      +61     
+ Misses       5785     5762      -23     
- Partials       83       85       +2     

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[maxlandon]

Superseded by #115, which merged this change (recommitted as a signed commit with your authorship preserved) into master via dev. Thanks for the contribution! 🙏